Well you have a point there. But these books seems like a kind of fundie Christian version of torture porn to me. It gives them the opportunity to get their rocks off watching/reading about a group of people they are prejudiced against get slaughtered for daring not believe the same things they do, while fantasising about watching it from their perch in heaven.
It's not about whether it's believable or not, but about why somebody would want to read about or watch that. I suspect it is simply to reinforce their own feeling of superiority.
Aren't most fairytales/fantasy stories, including the Bible, just morality plays and/or metaphors for the author's world view?
And most of those end up with at least some characters dying/suffering because of their failure to abide by the story's morality.
As such, there seems to be little reason to be more offended by "Left Behind" than any other fantasy story.