First, initiating an encounter is not the same thing as initiating hostilities. For a neighborhood watch volunteer to inquire why someone not from a gated community is walking through in the wee hours is not out of line.
The question is, with the boiling animus of racist victimhood percolating just subcutaneously, was the response to such an enquiry hostile and did it utilize force? One need not have a weapon to kill someone, blunt force trauma using hands and/or feet accounts for a significant enough percentage of murders every year, albeit behind the use of weapons, and repeatedly striking someone's head against a hard object will do the trick.
Cain smote Abel with a rock (the first recorded murder), and bringing the head to the rock instead of the other way around can be equally effective.
At that point, Zimmerman is under attack using lethal force, and to respond in kind, regardless of the means, could be justified if Zimmerman did not initiate the physical confrontation.
To assume guilt on Zimmerman's part (Because he survived the encounter? Because his skin tone is lighter? Because he had a technological advantage that permitted him to prevail?) is crap, especially when the facts have been carefully filtered by the media.
Zimmermann was also in lethal danger at that point, even discounting his account of Martin's threats. Losing consciousness (head on concrete) means losing control of his weapon to someone who's already working on beating him to death. "Use it or lose it". He was on the verge of going out.