Homo sapiens is the most destructive species, and vast amounts of this destruction are wreaked on other humans. Humans have killed one another since the origin of the species, but the scale (not rate) of killing has expanded (not least because there are now so many more humans to kill than there were for most of human history). The means by which many millions of humans have been killed have been dismally diverse. They include stabbing, hacking, slashing, hanging, gassing, poisoning, drowning, and bombing. Humans also visit other horrors on their fellows, including persecuting, oppressing, beating, branding, maiming, tormenting, torturing, raping, kidnapping, and enslaving.

The optimists argue that prospective children are unlikely to be among the perpetrators of such evil, and this is true: only a small proportion of children will become perpetrators of the worst barbarities against humans. However, a much larger proportion of humanity facilitates such evils. Persecution and oppression often require the acquiescence or complicity of a multitude of humans.