Check the FBI statistics. Of course handguns kill far more than hands, just making the point that the hands are deadly weapons, especially when they belong to those who are younger and on drugs. In this case, it appears TM was unarmed, but how many guys have been attacked like GZ was only to be killed via knife? Quite a few.
IOW, TM attacked GZ and was beating him, fairly badly by what the evidence shows. How do we know TM would have stopped before killing GZ? Zimmerman certainly didn't know that.
The takeaway here is you probably shouldn't attack someone if they aren't attacking you, and if you decide to attack someone physically, you better be certain they're not armed.