I'll bet Zimmerman was thinking the same thing. Which do you think he wishes he had used now?
Don't get me wrong, if any had shown evidence of a weapon or intent to kill (unless the biting was canabalism, I don't think it counts) -- I'm fully in favor of a kinetic solution to the problem, but I don't think that meets the criteria here. Anyway, I like to have ..... options.
I think that someone attempting to bite off your hands, or anything else off is “severe bodily harm” and would justify using deadly force to halt the attack.