I go both ways. I think there is something to be said for a clear motive in complete opposition to the hero, one where the audience can identify with the villain. When done well, the audience starts to identify more with the villain than the hero, which can be pretty disturbing. We see this as a trend (although not a new one) with Walter White and Dexter. Although, the first one that comes to my mind is King Lear. Bad guys the audience is pulling for. It is all the creepier because we identify with these villains and want their redemption so when they fail it feels like we’re failing, too.
Then again, I think about Willow’s fall in Buffy Season 6 (and again in the comics). She has a legitimate grievance the first time, but then she loses complete control of herself and just wants to destroy everything for the sake of destroying the world. Of course the Joker does the “I want to watch the world burn thing better” because of his seemingly lack of a conscience to negotiate with.