If you're grappling with a guy who *wasn't* subdued by a tazer, odds are that the guy is probably so amped up that he can fucking kill you, especially in a grappling situation. Guy just needs to get you in a position where he can twist your gun, bite your neck, or get a thumb in your eye and you're dead.
I'm FOR police increasing their risk level where possible to prevent death, and think that currently US policing has problems with being too risk-averse, but fucking hell. Anyone who thinks there's a "shoot first think later" attitude from police in this instance has no objectivity.