It's a simple action for a soldier to kill another human being and without consequences - at least not immediately. You see, the act itself is without emotion and devoid of much thought. The preparation for such a task is unconscious, but very important indeed. Soldiers generally are not willing to take a life, even in self defense. It is something that is incorporated in their training, subtle though it may be.
It all starts with anger, or revenge. Feelings not felt during the actual event, but begin the thought process. This emotion is not trained, but is certainly exploited. Soldiers are fundamentally angry at their enemy simply for causing them to leave their families. But, whatever the cause, all soldiers find some way to be angry.