It's because religion is a paranoid phenomenon that is based on dividing the world into good and bad.  You have to fight for the good and fight against the bad.  It will inherently cause violence.

Why do you promote violence er em murder?
http://answermug.com/forums/topic/17309/i-am-mystified-as-to-why-people-are-offended-when-i-say-trump-mu/view/post_id/130809

I am mystified as to why people are offended when I say Trump must die and soon. He is doing it right under our noses. He is destroying us.

It doesn't. People use religion to cause fear. 

It does when it turns into fanaticism. Goes for religion as anything else in life, Shraonna.

Religion doesn't cause violence. People cause violence.

I don't think religion itself is the direct cause.
Rather some religions are used as propaganda for the purposes of promoting war,
and for helping soldiers face their death (mind-washing for control.)
Not all religions, but often Christianity, Judaism, and Islam used as the excuses for war to gain things like land, resources, and power.
Also used to keep power and wealth in the hands of the elite -- OT promotes beliefs perfect for controlling slaves.
In the Bhagavad Gita, Krishna convinces Arjuna that dying for a worthy cause is not death but the means of earning a better incarnation, of evolving towards enlightenment and final freedom from the body and the suffering of life.
Buddhism was used by Ashoka to help conquer Northern India, C4th AD (CE).
Confucianism used for centuries to control China. Zen to control Japan.

This post was edited by Benedict Arnold at December 19, 2016 11:45 AM MST

Religion itself is not the cause of violence. The people who misinterpret religion are the cause of violence.