But if seven out of ten humans can't pull the trigger on an enemy they have every reason to believe is trying to kill them, it seems unlikely that ninety-seven out of a hundred could make themselves murder. Furthermore, all but a vanishingly small number of murders are performed by males between the ages of 15 and 25, and the overwhelming majority of those by unmarried males. One's odds of being killed by a human outside that demographic bracket are comparable to one's chances of being killed by a lightning strike. War is the great exception, the great legitimizer of murder, the one arena in which ordinary humans routinely become killers. Both war and genocide depend, critically, on the habit of killing on orders .
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. 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. Both, however, require the obedience of a large supporting population. Factories must work overtime.