It was dark, raining and Martin was walking near the homes looking in - that is in evidence - this area was a high risk area for burglary - that is in evidence - Zimmerman saw this man (Martin) lazily walking through the community looking into the homes, stopping and then continuing on. If he was going home why would he be checking out the homes? Martin's behavior was suspicious.
The behavior before the confrontation is all irrelevant. That all stopped when Zimmerman lost Martin and started walking back to his vehicle. At one point Martin's girlfriend said he was "at home" but the confrontation happened back within 30 feet of Zimmerman's vehicle. Who was stalking whom? Martin was never hit by Zimmerman - there was no indication that Martin had any injuries other than his bruised knuckles (from hitting Zimmerman) and the gunshot that could have saved Zimmerman's life.
Zimmerman had a broken nose, impact cuts on the back of his head, and bruising to both sides of his head. Zimmerman was pinned to the ground, unable to effect a withdrawal and was unable to fight back (as evidenced by no damage to Martin). Zimmerman would be a total idiot to think he was not in grave danger but he wasn't an idiot and recognized that this was a grievous situation and when his attacker noticed the gun and Zimmerman was told that he was going to die he did the only thing he could have done. He defended his life with the only effective means at his disposal. He fired his gun in self defense.
No sane person under those conditions would have done anything differently and survived. The gun was fired while Martin was bent over Zimmerman at a distance of four inches (all in evidence) and there is no way you could get the same powder residue or combination of angles in any other position.
There simply is no evidence that shows any other course of events is probable.