For those who say this movie shouldn't be considered offensive
First, the story in once sentence:
Intelligent drama about a boy trapped and powerless as a murder-accessory to his mentally ill, devil-believing, murderous father then morphs into absurd thriller about hunting and killing 'real' demons, implying that the boy's bloodthirsty father is a hero, not a mentally-deranged serial killer.
Ew.
This wouldn't be a problem if it was about vampires or zombies, because these character don't belong to a larger belief system that suggest these characters could be real. Instead, millions of Americans believe in some or all parts of Christian theology, which includes the devil and his demons.
To be clear, I'm not offended by this movie. I'm British and as a far less pious country, we don't have this problem politically. As you can see in public debates, devil-believers are laughed at and mocked. Thank godness. But the justification for claiming offense - using the rationale I suggest above - is entirely appropriate for the American public. Fact and fiction, where blurred in the eyes of the public, must be delicately handled. This film fails to factor that.