Reported someone for showing up to work drunk. He was fired shortly after. It was justified because in his job, he put others at risk of harm in addition to himself, but it still feels bad.
He was an old guy without a lot of employment options, so getting fired likely had a very negative impact on his quality of life. I know he did it to himself, but it's the only time where a decision I made was a clear link in the chain of someone else's life getting significantly worse. It was necessary, but still sucked.
I reported a coworker for being drunk at work 3 days a week...it took them 13 years! Until they passed him the ultimatum: either you accept our help with your alcoholism or you're out.
He left and started a new life in a new country, wife kids etc donated a large part of his money to a local school...only to die months after due to stomach issues as a result of years of alcohol abuse.
We had 2 guys like that and they took 13 years with one and 15 with the other.
The one of 15 years controlled heavy machinery daily.
I stuck all their Nazi faces all over social media and outed every last one of them as the Nazis they are to their jobs. Then I catfished a really big name in white supremacy and got him to send me dick pics.
When my boss was complaining about not getting matches on Tinder I told him he has manager arms. When he asked what that meant I told him it meant that they were non-value-add.
In middle school, during history class, we had a student teacher who was very insecure, and my class bullied him a bit. During a class where he was being evaluated, I went out of my way to disrupt his teaching and likely played a significant role in him failing the evaluation.
I was once on a secret mission in the statue of liberty and I was tasked with taking out a leader of terrorists, I killed the entire group because cmon they were terrorists, only to find out later that they were trying to distribute an antidote to a virus that the organization I worked for help distribute!
Helped a friend craft a set of emails a few days before the submission of a group project that got him to leave one group and create a new one. This worked.
It helped my friend out, but it left the rest of the group scrambling.