I'm a bit confused as to who it is supposed to appeal to. Is this whole series created just to allow Christians to feel smug and get their jollies watching atheists and muslims slaughter each other?
Doesn't sound very... Christian.
It appeals to Christians. Many Christians like these stories, like the way other people like disaster movies, scary stories, and post apocalyptic stuff. They also get the bonus of it not attacking their beliefs.
Most Christians I know are nice people.