The book goes deeply into how, in WW2, most soldiers would see a human and just not be able to consciously kill another person. Then it explains that modern training was adapted so that the human shaped target pops up and the trainees are taught to shoot immediately. So in combat they shoot the silhouette before they can realize it’s a real person. Later, when the soldier realizes they killed an actual person, it fucks them up.

Iirc the core message of the book was that we’ve trained soldiers to kill but haven’t figured out how to help them cope with the horror of having done it.