Why was Hitler so mean and hateful toward one group or another? I find it hard to believe he woke up one day and said you and you suck but these people over here are good. Taking it so far as killing?

Respectfully, do some reading of even middleschool level history books. He didn't personally decide to "be mean" to particular groups. His worldview was shaped by the post WWI dynamics in Europe, latent stereotypes and domestic problems within Germany, and a whole lot of misapplied emerging science. He literally believed in racial supremecy and was trying to build an ethnostate. This motivated him to define groups of ethnic "undesirables" and also to lump in other groups that he found politically inconvenient. Read about the scapegoating of Jews circa that time. He likely believed his cruelty towards them was somehow justified because he laid all of Germany's problems (economic issues, humiliation of the country and loss of territory post-WWI) at their feet.
TL; DR - Bad actors believe their evil behavior is justified.

There's probably as many theories as there are people.
My own is that cognitive dissonance is a powerful force, especially for a narcissist. Hitler spent 4 of the best years of his life is the pure misery of the trenches of WWI, and even worse - running between the trenches to deliver messages. He was gassed so badly it put him in the hospital for the final month of the war. Hitler was probably also deluded about Germany's failing position in the war, probably kept in the dark by German propaganda and made quite gullible by his fierce patriotism. So when Germany surrendered, that must have actually both shocking and enraging for him. And we know that because he frequently attacked the treaty of Versailles.
The "stab in the back" myth is the idea that Germany's surrender was corruptly carried out by politicians, and due to common antisemitism across Germany and Europe generally it soon focused on Jews. In reality there was virtually no Jewish people significantly involved with the surrender and events leading up to it, but that didn't matter because as is often the case today, emotional truth is more important than reality. Given that Hitler couldn't possibly bring himself to believe that he incurred tremendous pain and suffering by voluntarily and stupidly buying into an imperialistic war that was doomed to failure from the start, the stab in the back myth would've been extremely appealing to him. He wanted to believe it, so he did.
So when he happened to be assigned to monitor the DAP - the predecessor to the Nazi Party - he was ready to wholeheartedly accept their antisemitic ideas. Even though he seemed to get along with Jewish officers during the war, by a year after the war he was already talking about "removing" the Jews.

IIRC during the third Crusade, the Christians slaughtered 60,000 Jewish people before they even got out of Germany. The beef goes way back.
Some people will say the origin is Christ's death. But even reading that story it's clear that Europeans and Israelites weren't on good terms then either.
It's hard to find a "cause" for a belief that isn't rational.
- Pogroms were common throughout Europe for hundreds of years. Because Christians were forbidden from lending money at interest Jews were the ones doing the banking.
  When a ruler got too far into debt they’d incite some religious hatred and miraculously be out of debt, because their creditors were dead or missing.
  
  "Expelling the Jews" is a mechanic in Crusader Kings 2. And it gets abused like it did historically, too- borrow a shitload of money and then expel the Jews. Suddenly you found free money.
- Some people will say the origin is Christ’s death. But even reading that story it’s clear that Europeans and Israelites weren’t on good terms then either.
  Further, early Jewish Christians were persecuted by the Pharisees and other Jews who were angry with their breaking Moses' Law. It was probably only natural to gradually amp up the anti-Jewish sentiment as the Gospels were written because there were open beefs happening in daily life.

Scapegoating minorities was a tool for him to use to get into power, given that most people in Germany distrusted Jewish people at the time and coming off of the great war with massive dept and a broken country, someone would've taken advantage of the situation.

Hitler’s antisemitism was shaped by the widespread anti-Jewish sentiment in Europe, particularly in Vienna, where he lived as a young man. He capitalized on these prejudices, blaming Jews for Germany’s defeat in World War I, economic hardships, and other societal issues. By using Jews and other minorities as scapegoats, he was able to unite and mobilize his followers.
In Mein Kampf, Hitler described history as a racial struggle, promoting the idea of Aryan superiority while portraying Jews, Slavs, Roma, disabled individuals, and others as threats to his vision of a "pure" German society. His ideology was influenced by pseudoscientific racism and eugenics, which he used to justify policies of persecution and genocide.
Nazi propaganda reinforced and amplified these ideas, systematically dehumanizing targeted groups. This allowed for widespread discrimination, persecution, and ultimately, the Holocaust. Hitler’s actions were not impulsive; they were deeply rooted in his ideology and strategically used to gain and maintain power.

When a child first hears an adult express a racist attitude they are confused and don’t know what to think. They may not even understand who is in the group being maligned or even have a concept of groups. And even if they do, they may think “huh? I didn’t realize they were all bad.” But like a lot of things you hear as a kid, it just kind of sinks in.
And that seed has been planted. From there, confirmation bias does the rest. Anytime that growing person sees a member of that other group doing something wrong, they notice it and think “hey that’s what Dad was talking about.” Any such missteps from a member of that group get assigned to the whole group. Of course, people not in that group are judged as individuals. Classic in-group / out-group thinking, which is universal.
By the time the person is grown up enough to think properly, they may have accumulated lots of these moments of confirmation bias. By then it’s likely that they start assigning blame to this group by default. Litter on the ground? Probably one of them. Something got stolen? Probably one of them. And of course if race relations are inflamed in general there may be plenty of active fuel for the fire. A gang of “them” beat up one of “us.” Now it’s basically war, and the person we’re talking about didn’t have to do anything or be a direct victim of anything. It’s all just socially transmitted and then reinforced through observation with cognitive bias. Incidentally, this is why we should never stop talking about racism openly, because that’s the only way to interrupt this process going on in the back of peoples minds. Yeah, sorry, Morgan Freeman.
I am with you as a fellow lazy person who can’t spare the energy. But I can see how easily others slip into it. Hitler of course used it to galvanize people and turn that in-group / out-group energy into a political force.
- Wish I could upvote this more than once

There was a large economic element to him persecuting the Jews that gets ignored in normal discourse.
He wanted their assets to shore up German capital among other sources like slave labour at industrial facilities.
Nazi nepo babies are benefing from it to this day but everyone ignores that the same way they ignore how a lot of US capital was extracted from slavery.

Fascism needs a minority group to oppress, someone else to blame for all the issues makes everything easier. A common enemy also helps unite the others. Look up Umberto Eco, he wrote an essay on how to spot a fascist.
The cult of tradition. “One has only to look at the syllabus of every fascist movement to find the major traditionalist thinkers. The Nazi gnosis was nourished by traditionalist, syncretistic, occult elements.”
The rejection of modernism. “The Enlightenment, the Age of Reason, is seen as the beginning of modern depravity. In this sense Fascism can be defined as irrationalism.”
The cult of action for action’s sake. “Action being beautiful in itself, it must be taken before, or without, any previous reflection. Thinking is a form of emasculation.”
Disagreement is treason. “The critical spirit makes distinctions, and to distinguish is a sign of modernism. In modern culture the scientific community praises disagreement as a way to improve knowledge.”
Fear of difference. “The first appeal of a fascist or prematurely fascist movement is an appeal against the intruders. Thus Fascism is racist by definition.”
Appeal to social frustration. “One of the most typical features of the historical fascism was the appeal to a frustrated middle class, a class suffering from an economic crisis or feelings of political humiliation, and frightened by the pressure of lower social groups.”
The obsession with a plot. “Thus at the root of Fascist psychology there is the obsession with a plot, possibly an international one. The followers must feel besieged.”
The enemy is both strong and weak. “By a continuous shifting of rhetorical focus, the enemies are at the same time too strong and too weak.”
Pacifism is trafficking with the enemy. “For Fascism there is no struggle for life but, rather, life is lived for struggle.”
Contempt for the weak. “Elitism is a typical aspect of any reactionary ideology.”
Everybody is educated to become a hero. “In Fascist ideology, heroism is the norm. This cult of heroism is strictly linked with the cult of death.”
Machismo and weaponry. “Machismo implies both disdain for women and intolerance and condemnation of nonstandard sexual habits, from chastity to homosexuality.”
Selective populism. “There is in our future a TV or Internet populism, in which the emotional response of a selected group of citizens can be presented and accepted as the Voice of the People.”
Fascism speaks Newspeak. “All the Nazi or Fascist schoolbooks made use of an impoverished vocabulary, and an elementary syntax, in order to limit the instruments for complex and critical reasoning.”

You're trying to find a rationale for some thing (xenophobia) that is utterly irrational. Some people are just shit humans.
- He was a shit human, but he was also a politician. His political message had two parts: 1. Jews are responsible for all our problems 2. We need to conquer more lands.
  He said this was the only alternative to handing the country to communists.
  The simple xenophobic message appealed to the common people, fear of communism bought the elites in-line.

I recall he blamed the original surrender of Germany in WW1 on the Jewish people because the main signatory was Gustav Bauer, the chancellor of Germany who was Jewish.
- Gustav Bauer had no religious affiliation and in any case he did not sign the Treaty of Versailles. It was signed for Germany by Matthias Erzberger, who was also not Jewish. But, those sorts of facts tended to make little difference to the German far right of that time which basically branded everyone they didn't like as Jews.
  
  Bauer was Chancellor for Germany at the time and I recall Hitler attributing him to be Jewish. I think he made explicit reference to this in Mien Kampf but I cannot be sure as I have never read it. I'm not saying he was or that Hitler was right, it's just that he blamed the Jews for German defeat and troubles as a result of the treaty of Versailles.

Why was Hitler so mean?
Rough childhood driving him to resort to narcissism to escape depression.
Narcissism driving him to blame others for all problems.
Methamphetamine kicking the regular madness to the next level.
https://en.wikipedia.org/wiki/Adolf_Hitler#Early_years
[At the age of 6,] onset of intense father-son conflicts caused by Adolph's refusal to conform to the strict discipline of his school. Alois tried to browbeat his son into obedience, while Adolf did his best to be the opposite of whatever his father wanted. Alois would also beat his son, although his mother tried to protect him from regular beatings.
https://www.quora.com/Why-is-there-evil-in-people/answer/Harri-K-Hiltunen
... the infantile narcissism will be preserved as a kind of psychological fortress to protect the child against the vicissitudes of its intolerable life. Thus children may become evil in order to defend themselves against the onslaughts of parents who are evil.
... the longer we continue to make the wrong decisions, the more our heart hardens. ... with each step along the wrong road it becomes increasingly difficult for them to admit that they are on the wrong road
... When they are in conflict with the world they will invariably perceive the conflict as the world’s fault.
... As life often threatens their self-image of perfection, they are often busily engaged in hating and destroying that life—usually in the name of righteousness.
... it is out of their failure to put themselves on trial that their evil arises. ... We become evil by attempting to hide from ourselves. Evil originates not in the absence of guilt but in the effort to escape it.
https://en.wikipedia.org/wiki/Narcissism#Normal_and_healthy_levels_of_narcissism
... a moderate level of narcissism is supportive of good psychological health. Self-esteem works as a mediator between narcissism and psychological health. Elevated self-esteem, in moderation, supports resilience and ambition, but excessive self-focus can distort social relationships.
Destructive levels of narcissism ... feelings of entitlement and superiority, arrogant or haughty behaviors, and a generalized lack of empathy and concern for others.
https://en.wikipedia.org/wiki/Narcissistic_personality_disorder#Subtypes
... Grandiose/overt: the group exhibits grandiosity, entitlement, interpersonal exploitativeness and manipulation, pursuit of power and control, lack of empathy and remorse, and marked irritability and hostility.
https://www.psychiatrictimes.com/view/methamphetamine-dictatorship-hitler-nazi-germany-and-drug-abuse
Morell’s notes reveal that he put Hitler on a regimen of Vitamultin -- a preparation containing glucose, vitamins, and sometimes the methamphetamine Pervitin -- and prescribed dietary restrictions, bloodletting, leeching, enemas, and the morphine derivative Eukodal to relieve the GI symptoms. Sedatives (for sleep) and testosterone (for sexual potency) were also among the considerable number of drugs (over 40 different kinds) Hitler took over the course of his adult life.
Does methamphetamine use increase violent behaviour? Evidence from a prospective longitudinal study
Conclusions: There is a dose-related increase in violent behaviour during periods of methamphetamine use that is largely independent of the violence risk associated with psychotic symptoms.
- This look botty.

I think nobody mentioned this simple fact: the guy was pretty much mad

Some 1900 years earlier, the Roman Empire was having problems with the inhabitants of one of their recently conquered territories resisting their authority and decided to make an example of them by expelling them from their territory. They ended up scattered all over the Empire and beyond, whilst maintaining their language, religion and customs. A few hundred years after that, Rome adopted a new religion, worshipping a guy they had put to death a few hundred years earlier in that particular territory, and had to find someone else to blame, and the people they expelled were the most expedient candidate. And so, we had a millennium or two of European Christianity very unsubtly stating that the Jews were the killers of Christ, which got embellished, as urban legends do, with a number of other equally lurid myths about them poisoning wells with the Plague and abducting Christian babies for their blood. People generally (with exceptions) stopped believing in those myths, but the underlying attitudes of suspicion remained ingrained in culture.
- Don't forget that for centuries Christianity considered lending with interest to be a sin and so delegated it to Jews so they came to dominate banking and finance. By the 1100's Jews were considered the 'property of the king of England' and we're granted special rights such as free travel and having their legal testimony given more weight than Christians because they represented the king in financial matters.
  Elevate an already hated minority to wealth and privilege as special servants of feudal power? What could go wrong?
  
  It’s a trick the English repeated centuries later by establishing minorities as administrative castes in their colonies (i.e. Indians in Uganda)

Dont forget about the insane shit the Japanese did too. This dives into the reasons they did Mark Felton documentary: WW2 Japanese Military Brutality

As an authoritarian with an economy and power based upon the willing participation of the people its necessary to have a group u can blame for when shit inevitably goes wrong.
Every politician blames a group (that isn't themselves) for their failings. Trump blames the immigrants the democrats blame racist voters.
Going to the level of mass killings that requires some seriously fucked up shit in ur childhood to permanently fuck u in the head.
- Adolph's childhood would certainly be described by nobody as a good time. The one story I recall was him stripping naked to try and squeeze through the window bars in order to escape his father's wrath, only to get stuck. He found the humiliation of his father's laughter worse than the beating he was expecting.

Did nobody mention Nietzsche? Nazi worldview was largely inspired by twisting out cherry picking concepts from his works. The criticism of Christian religions idealizing the meek and looking back towards ancient Rome and Greece for strong, powerful, heroic characters and gods. The ideas of existence of superior and inferior humans. Etc.
- The eternal enemy of Dudes and Dudeism everywhere: Nihilists and Nihilism.

The Dude abides. ✌️
Much like the Dude himself, who is surrounded by people who are way too wound up in the world around him, you gotta remember. Sometimes, there's a lot of weird ones, like sexy feminists who swing naked from ceilings to paint, or your whacked-out 'Nam vet buddy who's just never moved on. And then there are assholes who piss on your rug, toe-severing German Nihilists, and rich old geezers who concoct kidnapping schemes to hide that he's been stealing from impoverished kids. And, of course, fascists. Lots and lots of fascists.
Remember, the enemy of Dudeism is Nihilism. And fascists are, usually, either Nihilists, scammers, rug-pissers, or other zealots.
- You're finally in your element, Donnie

Worked the same for Hitler as it does for Elon en Trump. The love for hate, the need to blame, the hunger for power.

The way i understood it was that at the time Jews did own a lot of wealth, much more than other people, and saw themselves as some kind of "better" or "elite" class. Many people did not like that, and that is the major reason why it was so easy for Hitler to find a lot of people to follow him.
- No dude, please don't spread the neo-nazi's myths for them.
  Your comment reeks of "they were snobby elites and are therefore partially at fault for their own persecution."
  Even if I accepted the first statement as true, your insinuation that jews in Weimar believed themselves to be of a superior class after they have already been the subject of discrimination for centuries is ridiculous. On top of that, your final statement that this is "the main reason" for the rise in antisemitism in the third reich is frankly disgusting.
  
  I'm not spreading anyone's propaganda, if that's what you're saying.
  I never said they were at fault for their own prosecution. That would be stupid to think.
  I don't actually know, and maybe the Jews were the good guys and unfairly used as a scapegoat for the problems at the time, or maybe not, i was not there, i have no way to tell. I only mentioned that they did own a lot of wealth, which I think can be objectively verified. That does not make them in any way inferior people though, not at all. I am not one of the "eat the rich" people that despises people simply because they are rich. Not at all. In fact, i highly admire jewish culture. I was merely trying to lay out an explanation. There can be no discussion when there is no hypothesis. No need to react so butthurt. Now, cheers!
- Not exactly (from my reading). The Jewish communities tended to be fairly insular, and focused a lot of their business inward. They still dealt with outsiders, but money flow tended to move mostly in one direction, so it seemed like the Jewish people were much better off and "taking" from the rest, when the rest of the country struggled. It was really more that a marginalized community took care of their own. Shocking, right?
  Plenty of other communities do the same. It's still a bunch of scapegoating.

Ingrained racist beliefs, as well as a pragmatic 'those are a group that can't fight back if pushed against. I will blame them because I want the majority on my side.'
However one cannot overstate the role cultural antisemitism played. Hitler didn't 'one day wake up.' Europe has had a long history of hating the jewish people. Scapegoating and Persicuting them. Blood liable for example was an accusation that popped up from time to time. Kidnapping people to mix their blood with matzah bread. Complete bunk, but the jewish people are a stateless people who do not share local traditions and norms nor do they worship at the same place. Plus they often found work doing the things that people either couldn't or wouldn't do, which lead to oft conspiricies of jewish money lenders puppeting from the background.
That is the sort of enviroment young Adolf grew up in. Same as how people in the modern US are essentially being taught to think of disadvantaged blacks and mexicans as a existenital threat sucking wealth away from them.
- To add on there is a documentary on Netflix called the circle of evil that dramatically shows the way hitler came to power. It’s not just antisemitism in Europe it’s a party of echo chambers with anti semitism and Hitler was the one most radical in his speeches while being highly influential. You don’t convince a country without a good indoctrinating speech about how the Jewish population is ruining everything. Germany just came off WW1 and a lot of bullshit stigma was thrown at the Jewish population because the war torn population stereotyped them as “helping both sides”. A lot of factors that nobody will be capable of recognizing are crucial to Hitlers power.
  
  You should listen to the English translations of the speeches. History doesn't repeat but it sure as fuck rhymes.
- A lot of Hitler’s ideas about Jews came directly from Henry Ford
- Also if you're wondering why any person turned out the way they are, looking at their childhood explains a lot. Hitler, in this case, had a violently abusive father and IIRC a few of his siblings died (I think from unrelated causes?) when he was a child.

Approaching this cynically, all right-wing politicians blame groups as others as being the cause of their country's societal ills. That's just how it works.
Jewish people were just one of the targeted groups, along with LGBT people, disabled people, and communists.
One universal truth throughout history is that it's hard to unite people behind a common goal, but it's easy to unite people against a common enemy.
Listen to any right wing politicians from around the world - their plans for making things better all start with getting rid of someone (be it a group of people or a governmental department).

If you have some time check out the Hardcore History Addendum episode "Superhumanly Inhuman" that delves into the nazis and the Holocaust. I just started it a few days ago
- Damn, I thought the Holocaust started in the 1930s! Shows what I know

woke up one day and said
LOL No. He built this up his whole life. Even wrote a detailed book about his ideas and plans 20 years or so in advance.

He wanted to be an artist, originally. His stepfather wasn't having it, he was rejected from art school, and his step harshly physically punished him while a sibling escaped that (I have a hypothesis on other abuse for that sibling but there are no known facts, so it's irrelevant). Women weren't valued either. I firmly believe a different childhood experience may have had a different outcome. Additionally, plenty of celebrated artists also broke rules of art. We won't ever know for sure.

I can't be arsed going into detail, but I'll give you a half-answer to get you started: "Negative integration". It basically means that ones identity is based on who you aren't as opposed to who you are. Ramp this up to 11, and you further this negative integration into defining who ones enemies are, and that's a core tenet of how fascism takes root.