I'm really into Owari no Seraph/Seraph of the End, which has a lot of these themes. It's humans vs vampires and the manga/anime have a major character who was turned, but humans also have to be willingly possessed by demons to fight the vampires, so there's a lot of examining characters' relationships to their own inner troubles and the nature of humanity itself. There are two different prequel light novel series as well, which deal with demons and vampires respectively and are quite good.