The military has taken notice that, over time, and through the history of war, the vast majority of individuals refuse to shoot to kill. That means, instead of firing directly at an “enemy,” soldiers (used here to cover all members of the Armed Forces: soldiers, Marines, airmen and women, and sailors) would fire their weapons away from their “targets,” or pretend to shoot. One investigation found -- and these studies have been replicated -- that in World War I only about 5% of people shot to kill; in World War II, about 15% of people shot to kill. By the US war in Vietnam, the rate at which soldiers were shooting to kill was found to be 90%. Today, that number could be even higher.
What happened? Training evolved to meet the military's goals.
There is a science of teaching soldiers to kill and it is called killology.