I actually think that unless you're writing unashamedly genreish fiction (and I love genre fiction with a mad passion, so that's not a criticism as such) the antagonist should never be a villain; or shouldn't be conceived as a villain, anyway. If the antagonist is a person, you should still get where s/he is coming from, even idenitfy with him or her. Tinfoil villains made of nothing but evil are just boring, to me at least.

I guess I try to write my villains in a way that, if they were the heroes, the heroes would be the villains of the story, if you know what I mean. The antagonist has to be in opposition to the protagonist but that doesn't necessarily imply evil. Even if they are "evil", the evil doesn't come from nowhere. I want to know why they eat babies, and "they're tasty" is not quite enough of a reason. A little bit of this approach, I think, helps in genre fiction as well, unless you're writing comedy/parody.