Colonel Grossman made this point in On Killing, that human beings are naturally reluctant to kill one another, such that most soldiers on the battlefield throughout history have gone to great lengths to avoid stabbing or shooting one another.  But this is only a half of the story.  The full truth goes like this:  Human beings are naturally reluctant to do violence to one another face to face, but any human being is extremely apt to do violence to another human being who is on his knees, or who has his back turned, or who is running away.  There are in human history very few warrior societies, but societies of what we in the modern West would consider unimaginable cruelty are very common, perhaps even the norm, since the dawn of civilization.  Unimaginable cruelty is not something you can do to another person in a stand-up fight.  It is something you can only do to a person who is at your mercy.

Indeed, soldiers throughout history have struggled to kill the enemy in battle.  They have much preferred to kill and torture prisoners, women, children, and slaves, and this a large plurality (at least) of the human species can do with ease and readily.  You, too.  This is your nature.  This is why experiments which test to see how readily people become cruel when placed in positions of power always find that for nearly all participants the answer is “very.”  It is human nature, and it is your nature.  The person who claims she could never shoot an attacker, that it’s just too violent, might indeed be speaking honestly.  What she leaves out, and dares not admit, is that she could much more easily torture him if he was captive and bound.