From The Myth of Man the Killer:
Human beings are social primates with social instincts. One of those instincts is docility, a predisposition to obey the tribe leader and other dominant males. This was originally adaptive; fewer status fights meant more able bodies in the tribe or hunting band. It was especially important that bachelor males, unmarried 15-to-25 year-old men, obey orders even when those orders involved risk and killing. These bachelors were the tribe’s hunters, warriors, scouts, and risk-takers; a band would flourish best if they were both aggressive towards outsiders and amenable to social control.Over most of human evolutionary history, the multiplier effect of docility was limited by the small size (250 or less, usually much less) of human social units. But when a single alpha male or cooperating group of alpha males could command the aggressive bachelor males of a large city or entire nation, the rules changed. Warfare and genocide became possible.
Actually, neither war nor genocide needs more than a comparative handful of murderers — not much larger a cohort than the half-percent to percent that commits lethal violence in peacetime. Both, however, require the obedience of a large supporting population. Factories must work overtime. Ammunition trucks must be driven where the bullets are needed. People must agree not to see, not to hear, not to notice certain things. Orders must be obeyed.