How religion become so important that it can make people kill each other?

Answer 1

It's not the religion,it's just the person.

Answer 2

I think it begins in our childhoods, depending on if and how your parents choose to expose you to religious beliefs.
As a child, a belief system cements their identity in a discipline of a doctrine(s). The more fanatical the doctrine, the less empathy and compassion for others beliefs, sometimes to the point where other beliefs represent malevolence and or evil. So a person with an extreme fanatical belief system , actually believes their doing good by killing something evil! I would also categorize serial killers, sexual pedophiles and crimes of morality in a similar context, except these people are raised by people that lack the ability to nurture, show affection and love, so empathy and compassion for others never develops, in both cases though these kinds of people are made, not born that way!