Some kill because their faiths explicitly command them to do so, some kill though their faiths explicitly forbid them to do so, and some kill because they have no faith and hence believe all things are permitted to them. Polytheists, monotheists, and atheists kill - indeed, this last class is especially prolifically homicidal, if the evidence of the twentieth century is to be consulted. Men kill for their gods, or for their God, or because there is no God and the destiny of humanity must be shaped by gigantic exertions of human will...
Men will always seek gods in whose name they may perform great deeds or commit unspeakable atrocities...Then again, men also kill on account of money, land, love, pride, hatred, envy or ambition.
Does religious conviction provide a powerful reason for killing? Undeniably it often does. It also often provides the sole compelling reason for refusing to kill, or for being merciful, or for seeking peace; only the profoundest ignorance of history could prevent one from recognizing this. For the truth is that religion and irreligion are cultural variables, but killing is a human constant.
David Bentley Hart, Atheist Delusions, p. 12-13