Frankly, I just like villains with controlled insanity. The Joker, Kimble from FMA: Brotherhood, Loki, Azula, Saren from ME, Meredith from DA: 2, Durza from Eragon (The book.), and Amon. They're all varying degrees of insane, they all have a variety of reasons for what they do, amd they all control their insanity to a point. I just like a villain like that. Clearly insane, but in control enough to go through with their plans.