My Dad served in World War II and was in North Africa and in France during D-Day. He never much talked about his experiences. My cousin served in Vietnam and he never talked much either. Whenever my Dad WOULD talk about WWII he mostly talked about humorous stuff, places he had seen, etc. I can't remember him talking about combat except he did say that D-Day was really terrible.
I do not approve of cold blooded murder. But in war every situation can be very different. For example, you might be on a patrol and you are told you cannot take prisoners. If an enemy soldier gets wounded, what do you do? Kill him or leave him alive to report your presence to other enemy soldiers? Leave him alive to be eaten by rats? Think about it. Finishing the enemy soldier off might be the most humane thing to do. I am not trying to be gross. But am I not telling the truth?
Think of the pressure these Marines were under. They are fighting an enemy who is more then willing to die, who will put explosives on the bodies of the dead, who will attack after showing a white flag of surrender, who will fake death to try to kill American soldiers, who will hide behind civilians. The Marine who shot the wounded terrorist saw that he had been faking death. The Iraqi might have been trying to do anything-explode explosive devices, pull out a concealed weapon, etc. Would you be quick on the trigger?
War is an ugly thing. It would be nice if all war could be ended. It has been said that every soldier who has ever really experienced warfare would prefer to never fight again (except for the crazy people, of course). We should not do things like torture and murder, but we do have to be able to defend ourselves. Don't send young men (and women) in harm's way if you are unable to deal with the consequences of war. I am not trying to defend the young Marine if he was wrong. If he was wrong he will be punished. You notice the enemy we face does not punish its member's who behead, torture, and murder other human beings.