if zim was a cop and he was in uniform and on duty and asked the kid to stop because he wanted to talk to him and the kid kept walking and he was like please stop and he walked and then they eventually came head to head and then out if nowhere the kid started just beating the crap out of the cop then I guess lethal force or whatever. none of that happened in this case. an adult male tracked down a kid in the dark. Idk but if I was a kid and some adult male(not cop) was following me and cutting through yards, is be scared as crap and would throw punches if the guy cornered me.
Add to that the fact that Zimmerman thought TM was subhuman evident by his own words, "
these assholes" "what are YOU doing here" Not saying its related to race, but he definitely was in hero-mode, ridding the earth of its scum.
With that frame of mind seems likely that when the encounter did happen that Zimmerman was in an aggressor state of mind. Even then, you can't blame TM for being the first to initiate the physical contact - for all he knew this sweating, clearly agitated man standing in the darkness had finally caught up to him.