Luca Blight.

Because he is pretty much the completely bat insane stereotype, but he actually has real reasons for being such a thing that would have messed anyone up. He is crazy, and he's mad with revenge, but he has reasoning for wanting this revenge so desperately.

As for what makes someone evil, I think Big D's sums it up. Actions can be impossibly evil but it's the motivations which really rank someone's place on the tree of evil.