You have a point. There's so many stories where I can't comprehend why the villain is evil (although when done well, an evil for no reason -villain can be the creepiest one). Being powerhungry, or bitter because of something that happened in the past work best for me... And villains who think they're not being evil at all, who can reason so well to themselves that they can justify their evilness, I find those ones to work really well.

Then of course there's the ones who are just insane. :3 Like Joker, I don't know anyone who doesn't like Joker :P