It's a big assumption that people wouldn't be just as good at finding other reasons to kill each other in the absence of organized religions - genocides by Stalin, the Khmer Rouge, Rawanda come to mind. Even the Holocaust, while it was directed at a religious group, was not a religious imperative of German Lutheranism. Religion often becomes the stalking horse to dress up more fundamental aims of conquest, greed, land, ethnicity. Still, if conquest were made a harder sell without the availability of the gloss of God's consent, that might be progress.