An Excerpt from Behave

in World War II only 15 to 20 percent of riflemen ever fired their guns. The rest? Running messages, helping people load ammunition, tending to buddies—but not aiming a rifle at someone nearby and pulling a trigger. Psychologists of warfare emphasize how, in the heat of battle, people don’t shoot another human out of hatred or obedience, or even from knowing that this enemy is trying to kill them. Instead it’s the pseudokinship of bands of brothers—to protect your buddies, to not let the guys next to you down. But outside those motivations, humans show a strong natural aversion to killing at close range.

Sapolsky, Robert M.. Behave (p. 645). Penguin Publishing Group. Kindle Edition.