Here’s an interesting article: http://www.patheos.com/blogs/secularoutpost/2014/09/02/religion-and-violence/ .
1) When someone challenges your religion, it’s easy for the believer to feel personally insulted. And that feeling is known to easily provoke a violent response.
2) Challenging religion may also be perceived as an attempt to dissolve societal norms and rules including morality issues. Such feelings often provoke more or less violent responses.
3) Since religion is what gives you a sense of meaning in life, then you’ll become more inclined to aggressively defend your standpoint, if someone tries to question your sacrosanct beliefs.
4) Religion is a way to divide people into two groups or categories: a) Those who share your values and beliefs; and b) Those who don’t. Such a dichotomization often leads to aggression and violence.
5) If your religious doctrines say that eternal punishment in Hell is the consequence of departing from true belief, then it’s sort of a divine duty for you trying to stop the spread of messages from so-called false prophets (and atheists), at any cost.