According to international laws the US sponsored and heavily promoted, and in fact constantly holds other nations to, it is illegal to kill an enemy soldier if he/she is a) unarmed and poses no physical threat, and

wounded in combat. That is a law of war and indeed of simply being a good human. It's set in stone. To kill someone in the a) or

manner is a war crime. And we all know it. No need for lame games here. If someone shot a US G.I. while he was lying helpless and wounded, you would all rightly call it a war crime. Here's the link to the uncommented-on thread I made a while ago.
Ok, well I'll argue with this. Not really that the Americans would be breaking the law as it stands, but that the law is actually just. It isn't. In war there are no rules. You are fighting a war because the agressor is out to physically harm you and isn't interested in civilization. Thus to allow any enemy that is trying to kill you to live, no matter what the scenario is ludicris.
You don't negotiate with terrorists because to do so would be to compromise with evil. Similarily you don't allow evil to live no matter the case. You kill, murder do whatever you want to call it, and go on killing them until you can be sure that they will lay down their arms and act in a civilized way, which is to say, do no physical harm to another unless that other does physical harm to them first.
This is why the US is back in Iraq. Instead of whiping them out the first time as should have been done, they left and it all boiled over again with terrorism etc. If they'd done the job and forced the issue by killing every last person that wanted to opress americans or even Kuwaitis this there would have been no need to go in a second time, and I dare say that 9/11 likely wouldn't have happened because just like this time, democracy would have taken hold and that would have been that. People like Bin Lauden wouldn't have been able to raise the money to do what they did, because they would have been marginalized by the great force that is freedom.