The whole reason one has a gun is to use it to keep from being harmed. If it was Martin that caused his injuries than it's reasonable to believe that Zimmerman had brandished or displayed the gun which makes Martin defending HIMSELF violently entirely justified. No one that carries a gun allows themselves to even get IN a physical altercation much less on the losing end. It was that gun that gave Zimmerman the bravery he needed to chase after Martin and confront him, and there's no question he would have pulled it out the second he felt in the slightest bit of danger from Martin. He killed Martin not because he was afraid for his life but because he was PISSED that Martin didn't meekly consent to whatever he wanted and may have gotten some licks in.
The defense counsel O'Mara made it clear in testimony with one of the officers that Zimmerman's gun could easily be seen by his merely thrusting the hip where the gun was holstered forward slightly which caused his jacket to open and show the gun. That's how he showed the officer on the scene where his gun was when that officer wanted to take if off him. O'Mara himself made it clear that Zimmerman's gun could be seen by his hip maneuver, which means that Martin may have seen it before Zimmerman decided to brandish it which also gives Martin reasonable justification for defending himself against a creepy belligerent dude that chased him and confronted him for no legitimate reason.
And of course Zimmerman wouldn't have shot him before getting into a fight with him. He'd have no injuries to explain self-defense and would go to jail for the rest of his life.