FF6 "Choose your own Adventure" style

I didn't think that I could make enough options where the main character would realistically do something evil.

Then you have not played SaGa frontier. Asellus, a half-human, half-Mystic, gets to decide her fate depending on the actions you take throughout the game. Choose wisely, and she becomes the next full-fledged Mystic. She overthrows the previous ruler, becomes the new ruler of Facinaturu, turns one of her best friends into a mystic(vampire), she gets 100+ lesbian lovers, and manipulates everyone to do her will.

Craziest ending ever IMO, coming from people like Square.