It has been noted that demons and angels are similar, but created in different ways. A demon by a wizard, and an angel by a god. So could it be that demons are just crude copies of Angels. If so it would explain why they dislike the ugly, twisted imitations of themselves. It would be like Frankenstein, but weaker and inferior to you in every way, but similar enough that you might feel uncomfortable. Some humans hate the idea of identical, or improved humans created by other humans, so Angels/ Gods might hate the idea of ugly, weaker, crudely made demons made by humans that just happen to resemble Angels. It goes against many of their ideals and could allow humans to glean more information about the origins of the celestial and divine than they might like.