There was a great interview I once read with Willem Dafoe (which I'm sure I'm going to paraphrase horribly) where he was asked how he felt about playing "good" characters compared to "evil" characters. Dafoe said something like "It doesn't matter. Everybody thinks they're righteous." And that's really stuck with me.
One simply cannot go wrong in taking a page from the Willem Dafoe School of Overracting.