So you would have done more than break his neck? You need to find a new job if you have to do more than break an old man's neck to put him in his place.
I'm pretty sure the cop didn't sneak around behind the old guy and snap his neck like a bad action flick. The old man proposed a fight and the cop accepted. I see intent, imminance, and weapon system (bare hands). I see no reason why the cop went beyond the pall by meeting the incoming force with a like level of force. The cop showed restraint by not only using like force, he gave the geezer three warnings. The cop made one move. If this old guy didn't bounce he shouldn't have picked a fight.