The severity or number of Zimmerman’s injuries aren’t a big deal. Once Trayvon started his assault, Zimmerman couldn’t know how severe it would get. And in a helpless position, he could reasonably believe that he could possibly be beaten to death.
And after already having had his nose broken and having been beaten and having been verbally threatened with death as well...when TM saw and reached for his gun, GZ KNEW he better get to that gun first...and use it.