Why do we humans kill each other? Many like to put the blame on religion and they point to examples such as Islamic (and Christian) extremists. I believe that's an unfortunate example of shallow thinking. Let's look back at some history.
What really motivated the Hutus to massacre million of Tutsis -- religion, or something more instinctive, like territoriality?
What about the genocide in Sudan -- belief in God, or something more basic, like competition for available resources?
What about the massacre of the Aboriginal population of Australia? In the 19th century, it was legal for settlers to shoot natives on the spot. Was this for religious reasons, or something more fundamental -- such as instinctive racial bias?
What about Bosnia and Croatia -- Godly motives, or unrestrained animal self-serving behavior?
What about the Holocaust? Would it have been prevented if Hitler had been an atheist -- or was the reason for the extermination of 11 million Jews due to something far more fundamental to human nature than paying lip service to a religion?
What about the genocide of Native Americans by the Europeans? What were they really after, my Cherokee grandfathers' souls -- or their land?
There are many more examples of humans committing atrocities against humans. It has been occuring since before H. saps saps wiped out H. saps neanderthalis. It occurs on all scales, from individuals against individuals to nations against nations. It happens because we H.saps have, like all animals, the instinct for survival -- and that instinct is best served by having all the marbles and eliminating all the competition.
Religion is just a convenient excuse some use to do what they would still do without it. If religion was not available, we'd use science (eugenics, for example), sociology/economics (Communism, for example), or nothing at all (I'm bigger than you, you have what I want, and I don't want you marrying my sister).