Why are demons in Diablo 3 considered "evil"?

The angels want to exterminate us. Nothing is more evil than that. The demons, though, just want to corrupt us.

I love to be corrupted.

Seriously. In the game, why are the demons "evil"? What do the demons do that's more evil than the angels?

What's the official "in game" explanation that the demons are evil and everyone considers them "evil"?

Note: This question is about angels and demons in Diablo 3. Not angels and demons in real life, which may not even exist.