His sneaking up seems to have been very rapid. A Combat experienced vet would have recognized it, perhaps in time to unlimber his own piece.
as a cameraman, i have learned to keep both eyes open as to see more than just what i am taping, as to not miss more interesting visuals...
it has also been said that more guns would not have saved the two victims... i disagree... someone other than the two victims might have been able to stop this jerk if they were carrying.