What the marine did was murder if the video is unedited.
I'm sure his mental state will come into play as a defense (as it should), but the act was no doubt a murder. If he is not a sociopath (like Timothy McVeigh), I think it might haunt him his entire life. If he is a sociopath (like Timothy McVeigh) I hope they find out before they turn him loose in America.
I hope people note, that there were many people there with guns and only one played God. The marines train soldiers, not murderers. The barbaric acts of a few shouldn't be used to smear all the good soldiers risking their lives over there for us.
Those in the forum who would defend murder, well all I can say is you indignation of terrorism is only tactical. You are of the same mindset.