The POV of the Marine.
Do we know what that Marine saw?
Did we see others feign surrender only to kill you buddies?
Did we see someone, who very well may have still been a threat to the Marine's personal well being, rolling off a IED trigger, pulling out a grenade, or some other slight move that could reasonably have been interpreted as a threat?
Perhaps it would be reasonable to withold judgement until more is known about the incident.