If the Marine thought the "dead" terrorist was booby-trapped or was going to try to kill him, the other marines and the journalist, then he did the right thing by killing him.
He could have saved their lives.
Actually, after seeing the *full* footage from the BBC link given above, I now have no problems with what our Marine did. Why? Because after our marine shoots the wounded guy, another terrorist in the room raises his hands from under a blanket and surrenders. At least one terrorist in that room **was** playing possum.