I agree. As Americans we are supposed to at least aspire to be on the moral high ground...and recognize the difference between killing in combat and the murder of a surrendering enemy...
Covarrubias, a 20-year Marine veteran, said he found the soldier inside a nearby house with the grenade launcher by his side. Covarrubias said he ordered the man to stop and forced him to turn around. "I went behind him and shot him in the back of the head. Twice," Covarrubias told the Review-Journal.
He just described the execution of a POW.
I would expect a 20 year Marine Corps GYSGT to know better.
It sounds like he is the only witness (still above room temperature). If he shuts up, he'll probably get away with it.