On Killing by LtCol Dave Grossman, 2009, Excerpts
In the modern bayonet charge, one side or the other usually breaks and runs before they meet, and the psychological balance tips significantly. When soldiers to turn their backs and flee, then the killing truly begins, and at some visceral level the soldier intuitively understands this and is very, very frightened when he has to turn his back to the enemy. The vast majority of casualties in historical battles were inflicted upon the losing side during the pursuit that followed victory.
The worst thing you can ever do is run from an animal. There is a chase instinct in most animals that will cause even a well-trained and non-aggressive dog to instinctively chase and pull down anything that runs. As long as your back is turned you are in danger. In the same way, there is a chase instinct in man that permits him to kill a fleeing enemy.
One factor that enables killing from behind is a process in which close proximity on the physical distance spectrum can be negated when the face cannot be seen. Not having to look at the face of the victim provides a form of psychological distance that enables the execution party and assists in their subsequent denial and the rationalization and acceptance of having killed a fellow human being.
Dragon Seed by Pearl Buck, 1942, Excerpts
If a man turned and ran when he saw the enemy he was shot where he ran and there fell thousands like that in one day. And if a man seemed at all like a soldier or as though he had been a soldier, he was killed, and there fell thousands like that in one day. And if any man was slow to move when he was given a piece of work, or if he was too young and not strong enough for a great burden on his back, or if he was an old man or a scholar who had never done hard work, then he too was killed, and so fell yet more thousands in a day.
Fall of Saigon