I'm not entirely convinced. Death may be preferable to killing a civilian ("I am an American, fighting in the forces which guard my country and our way of life. I am prepared to give my life in their defense."). But, that's an extreme knife-edge case that, if the defense could show at trial, should result in an acquittal - if the Marines would prosecute this at all. But, as sinky rightly observed, the Marines are acting with such a gravity that it is unlikely that even this occurred.
I would withhold judgment until I hear the story of the accused. There honestly are things on the battlefield that defy human ability to dream them up
Perhaps that would be the way you felt, but if the civilian was shielding a guy who was killing your buddies and who has vowed to kill you and your country, and the only way to stop him is to cut through that shield, then what are you going to do? Let your buddies die? Let the enemy win? Or do what is necessary to save your buddies and prevail in the war?
Perhaps you should not be a soldier. Bomber pilots often kill as many "civilians" as they do bad guys. But collateral damage is often a necessary evil.
BTW when the Israelites were ordered by God to kill all the Amalekites, if you had been there would you have shirked your duty, or would you have followed orders? They did what was necessary under the circumstances. The fact that they disobeyed God and did not finish the job came back to haunt them.