An old writing instructor of mine claimed that the most believable villains are the ones who really believe that what they do is good.

Write a story about a villain whose actions are viewed are evil in the eyes of the narrator, but who views himself as the good guy, whether he is misguided, acting on imperfect information, using a different ethical calculus, is just on the “wrong” side of a dispute that is not black-and-white, or for any other clever reason you can think of.