Some people claim that it is really, really difficult for humans to psych themselves up to kill another human. They often cite a claim by S. L. A. Marshall that only a small fraction – less than 25% – of WWII American combat infantrymen fired their weapons in battle.
Other people (Dave Grossman in particular) have built major theoretical structures on this observation, saying that humans have a built-in mental module than inhibits us from killing conspecifics (presumably for the good of the species). Grossman parlayed this line of thought into a stint as a professor of psychology at West Point.
Which is pretty impressive, especially when you consider that it’s all bullshit. S.L.A Marshall’s ‘data’ is vapor; there was and is nothing to it. He made shit up, not just on this topic. There’s every to reason to think that the vast majority of infantrymen throughout history did their level best to kill those on the other side – and there’s a certain satisfaction in doing so, not least because it beats them killing you.
You have to wonder about a universal human instinct that apparently misfired in every battle in recorded history.