Under the Geneva Conventions, it is illegal to kill a wounded enemy soldier who is not fighting back. The key information missing from this whole story is the convention applies only to UNIFORMED troops of a nation that actually SIGNED the conventions. Under the Geneva Convention, the Marine was well within the law to kill all the terrorists in the room. Under international law, we have NO obligation to care for these killers; in fact, we are specifically permitted to summarily execute them if captured. Distasteful, yes. A war crime? No way.
"Insurgents and terrorists don't play by anyone's rules but their own. They're not held to the same standards, as the Hague Convention applies only to state actors. Technically, I'm told, the United States doesn't have to adhere to the conventions when fighting non-state actors such as terrorists either, but we do "because it's the right thing to do."