To me the beating in the video wasn't about trying to kill the guy, it was about beating the hell out of him. If the guy would have dropped the big guy may not have kept striking, but he saw a weak guy and wanted to hurt him because he was still (sort of) standing.
So where is the line drawn between using deadly force if the victim has a fat lip and a broken nose versus the broken eye socket and pulverized face.
Making the right call on using deadly force in the defense of someone else is the clearest in hindsight. Hope to never be in that situation.