They are killing people because they believe their faith has been insulted. If they believe their faith is strong and makes sense, why can't it be questioned and even insulted? Who decided that it must never be questioned? Aha, it was god! But surely.... don't bother. They've been told it's a crime to question 'The Word'. Why oh why can't they see this is such a circular argument? 'Don't question it, 'cos god told the prophet what to say'.
If 'followers' asked themselves the simplest of all questions 'what if there's no such thing as a deity?'... or at the very least, 'what if our deity is not the right one out of the myriad that our species has invented since we began to question the world we live in', then they would see such 'teachings' or ideas might in fact be a human construct based on that circular argument - "God exists because he told someone he exists. We believe what that person says because god talks to him. We know that's true because he told us it was true." Take out the supercomputer in the sky bit, and we end up with nothing more than ideas - that are apparently worth murdering someone's children for.
It's very hard to be charitable to these people, even though from birth they have been indoctrinated with the concept of that superbeing who absolves them from attempting to solve the mysteries of the world for themselves, and who will punish them if they don't perpetuate and defend a belief in his existence. The only way to break this circle of blinkered thought is to help their children to recognise that holy books represent the start of human understanding, not the end of it.