The first person that came to my head was Eren Jaeger. To be honest, there are a lot of villains the choose from,I just chose the one I'm really educated in.
Why do you think Eren turned into the villain of his own story?
Attack on Titan was the story of Mikasa written in Eren's POV and told by Armin. That explains why at the beginning of its first season,we hear older Armin's voice.
Now,some would say Eren turned evil because he swore he'd wipe away all titans the day his mother was killed by one. I think the main reason why people saw him as evil was because they had no idea what he was going through. Yes,indeed everyone has a choice to either do evil or do good but sometimes no matter how much good you do in this world,it still remains the way it is, a very cruel place,this is something we all saw in the story of Attack on Titan. Eren wanted the world better, safer. He wanted a safe place for the people he loved. Now,yes we all know he used some extreme methods and killed over billions of the world's population but he didn't really have a choice. Plus,he said he was sorry🥹🕊️