The whole "atheists have killed more people than religion ever has" argument came about in response to the "religion is the cause of all wars" type of arguments used by atheists against Christians (and others).
Let's be straight here. There are a lot of very nasty people out there who will happily condone rape, pillage, murder or genocide if it gets them one step nearer their goals!
They will use whatever is at hand to accomplish this, albeit race, religion, politics, favourite football team or whatever. This is not the cause's fault, it is the fault of the individuals who misuse it.
Foxtrot is right that the root cause is a selfish concern for power, glory, money, influence, fame, etc. He is wrong to assert that Christians can't behave this way, or that atheists are somehow more predisposed to.
To reiterate a point I made elsewhere, it's not so much what people believe but their reluctance to honestly evalute those beliefs against reality.
Hermann Goering once said, "[T]he people can always be brought to the bidding of the leaders. All you have to do is to tell them they are being attacked, and denounce the pacifists for lack of patriotism and exposing the country to danger."
Atrocities, crusades and wars happen when influential individuals manipulate the natural fears of the people to the furtherance of their own ends. And the best way to avoid this is to encourage critical thinking, foster debate, and tolerate opposing viewpoints.
Theists and atheists alike would do well to remember this.
