That's a very tempting thing for my Hero game. Oo. New villains.
Evil isn't always Pure Evil. And Good isn't always pure good.
For instance, imagine a guy who wants to not destroy the world, but to change it. Because he sees a lot of things wrong with it, and when he shapes the world, they will be fixed.
This is a very Good goal. But what if what he wants, or sees wrong, is not exactly 'Good'?
And, another thing that makes Villains more real, is the fact that they hold things above their goals. Honor, respect, that sort of thing.
Who is the better villain, the one that is nearly zealotous in his methods, in his actions and goals, or one who does this: "Wow. You know, even though I want to rip you apart, even if I hate you all to the depths of my soul... What you just did, I respect. Take a breather. Drink a potion."
I think a party can respect a villain like that. Because they hold things to be more important; doing the good thing, fighting the good fight, because it IS the good fight, and they can see that the same goes for the NPCs.