Soldiers are not above inhumane and brutal actions. War itself is vicious business and it shouldn't be a shock when soldiers act viciously. That does not mean, however, that crimes should go unpunished.
It's a fact of war that one will likely end up dealing with people who were trying to kill you just a few minutes ago. How is a soldier supposed to react to the surrender of enemies who were just firing on him and who probably killed more than one of his friends? The laws of warfare require that he accept their surrender, but the facts of warfare are that many times the surrendering enemy ends up being shot right there. Understandable? Of course. Legal? No. Just because it's understandable doesn't mean that it isn't a war crime.