Religion is one of the leading cause of death and if you read the history most of the wars are based on religion. But I think its not that islam or muslim kill christian its that its our nature we kill each other regardless of religion.
Several of the largest genocides were undertaken by the U.S.S.R and the Communist Chinese. Both were devoutly anti-religious (although there is some evidence to suggest that certain of the communist leadership in Russia, at least, were into certain esoteric forms of spirituality with a roots in Kabbalah.)