Mark is absolutely correct in his reference to "On Killing". Throughout history, apparently, it has been difficult to force the common man (usually the larger % of "Armies")to take another man's life in Judeao/Christian cultures. Often NCO's must literally force their men to aim and fire. Often it is embarrassment in face of their brothers-in-arms that prevails, or the outright fear of death in the face of an overwhelming attack that forces response, and then often men dry-fire empty magazines. A Sergeant's job is not to participate singly, but to monitor his men assuring applicable defense or attack. Culture defines levels of acceptable violence. Poor leadership allows excesses.

I have no clue as to why some cultures are devastatingly violent. I think that in evolutionary terms we are but the blink of an eye past hunter-gatherer groups that defended, or attacked, good feeding grounds. I read somehere that man did not really achieve consciesness until about 5,000 years ago, but had become a hive-animal having recently (10,000 years ago)tamed goats and dogs. With that form of living an altruistic response (preservation of species, or group) was paramount.

Nowadays some cultures hold esteem for killing your father's enemy and his children, so they eventually will not become your son's enemy.