The objectively baseless moral reasoning of atheism can easily justifies atrocities as being reasonable solutions and means to their power hungry ends.
http://www.scholarscorner.com/apologia/deathtoll.html
As for “Christian” atrocities, whenever mortals presume supremacy over Scripture, then the souls as well as the bodies of men are in danger, as things from the carnal means of the Inquisitions to the Utah War manifest, in contrast to N.T. Christianity.