What's the worst twist you have seen in anime as far? Spoilers ahead

[DarkAngel]

i would say it's the twist of monster where that guy kills his wife and child for chimera creation.
i don't like it no matter what others say

 

[Kaname]

I would say Vampire knight anime. It's about Yuki's origin and her relationship with Kaname.
It's a spoiler.

 

[Grungie]

Hensuki. They were trying to go down the incest route, and then he had a flashback and remembered that she wasn’t actually his sister!

 

[Star_Of_Hope]

I watched too many anime with too many bad plot twists to pick just one here.
Probably I would go with Monster chimera scenario here or probably Bleach how Aizen planned everything from the beginning