Mordred, from Arthurian legend. If Arthur had embraced him as a son, been a real father to him, would Camelot have fallen? The whole thing is very "for want of a nail" and that's part of what makes it so tragic.
Also politically it feels like a really powerful example that repeats itself over and over of how good, idealistic governments in principle decay and fall from the inside from their own contradictions, but ultimately the killing blow comes at the hands of their own cynical and rejected offspring.
I love Mordred. In fact, I have an whole folder filled with "fanart" of Mordred
Clytemnestra. Agamemnon sacrificed their daughter, disappeared to war for 10 years, shows back up with a sex slave and is just revered as a hero. Fuck that guy. I'm glad she killed him. He deserved it.
I'm fairly partial to Baba Yaga in all her forms/myths.
Yes, her house hides all kinds of surprises
The Mordred answer is a great one. To add a new name to the list though, I'll pick the Korrigan. Old stories of them would eventually inspire Tolkien to write The Lay of Aotrou and Itroun, which is a delightful read about a couple that wants to have a child but struggles to conceive. The noble husband encounters a strange woman in the woods that offers to help, fuckery ensues.
Edit to add, just in case that doesn't scratch your mythology itch the right way, another great one: Set or Seth from the Egyptian pantheon
Another edit because I can't help myself: Enlil! He's from the Sumerian mythos, and in the Atrahasis story he is the god that decides to send a flood to wipe out humanity. Luckily, another god named Enki has a human he really likes that he gives a head's up to. Another fun story that shares some similarities with the Noah's Ark story.
I love just how simple yet captivating the Grim/Black Dog is. It isn't even necessarily aggressive, but just the mere sighting of it elicits dread and heralds doom.
Balor from Irish legend - who doesn't love a giant with a frickin laser beam for an eye
