As horrible as the video of the shooting Marine may appear, it is quite clear from the footage that a mistake of tragic proportions has been committed. Apparently the said Marine had been told or otherwise was under the certain impression that the #3 insurgent (from right to left in the video footage) in the room was dead. There were two known dead insurgents already in the room, and it is easy to confuse the status of the various bodies in the heat of a battle.

Now this marine was injured and another Marine next to him killed just the day before when an injured insurgent had exploded a suicide vest or a grenade. So there is real danger that an injured or dying insurgents would attempt to blow up Marines that come close enough. The suicidal tactics of these Islamic insurgents are legendary.

So the Marine unexpectedly sees the #3 guy move and gets the impression that he is feigning death and planning to detonate some kind of a device, possibly a suicide vest. That is when the Marine shoots the insurgent to prevent harm to himself and other Marines in the room. There is very little time to take this life and death decision.

This is further confirmed by the Marine's exlamation prior to the shooting that "he is f***ing faking to be dead" and after the event his remark "Sir that was a mistake, that was a mistake".