Kid: Who's the bad guy, Mommy? Lord Voldemort or Professor Snape?
Mom: Lord Voldemort is the bad guy. He's evil and does bad things.
Kid: But what about Professor Snape? He's not a good guy, is he?
Mom: Professor Snape is like a Republican. He's not evil, but he ain't great either.
Advanced Harry Potter readers, of course, know that the answer is considerably more opaque than that, even with just one book left to go. But, it is hard to imagine any more accurate answer in two short sentences or less.