I rather like SkinWalkers point that ideologies drive people to mega-murder. Those ideology may be religious, secular, or atheistic (i.e. anti-religious). It seems to me a futile argument whether it is religion or atheism that motivates mass murder, when both can (and have been) twisted to that end. It is the act of twisting them into a fanatical ideology that makes them dangerous, not a/theism itself. You can misuse anything with enough determination. Is science evil or good? You can use it to make WMDs or medicines.