In the book/movie they actually do kill God. Blasphemy is frowned upon in Christianity from what I've seen, so that was probably her major issue with it if I had to guess.

SPOILER:

A simplistic explanation is that, in the story, God is just the oldest angel and came into being before all other sentient life. He then convinces all the other angels that he created them (and everything). In actuality, he just happened to come into being first, and all angels are formed of a particle colloquially referred to as "dust," which is both the byproduct and catalyst of sentient life.

It's actually a really good book series, the main characters are children but the story is honestly more written for an adult audience.