I believe Zimmerman unwisely persued Trayvon and had already classified him as a bad person. Trayvon lost Zimmerman on his way toward his dwelling. At some point, Trayvon, unwisely, doubled back, while still on the phone with his girlfriend yet, well within his rights, given the proximity of his dwelling to the shooting.
All this while Zimmerman, continued to look for Trayvon, for a full two minutes after he said "ok", to stop pursuit. He could have easily been back to his truck had he truly returned toward his vehicle, as Zimmerman claims, and they never could have met where they did on the map.
They met face to face on the sidewalk, next to where Trayvon's body was found. Trayvon says, "Why are you following me?". Zimmerman responds, "What are you doing around here." and a fight breaks out. We will never know who through the first punch, and honestly it dosent matter. If the police report is accurate that Zimmerman had blood coming from his nose and some superficial scrapes and/or bumps on his head, that would be consistant with Trayvon punching him in the nose, at which point they began to wrestle, causing Zimmerman to hit his head on the concrete.
Visuals so far are not consistent with having your head severely bashed into the concrete, plus, it was raining and Zimmerman is bald.(Kind of hard to hold a bald head, in the rain.) The wrestling match worked its way into the grass where at some point, the struggle for the gun began. With the pistol holsterd down the front of Zimmerman's pants and Trayvon of top of Zimmerman as described by the witness "John", Zimmerman attempted to reach for his gun. At which point, Trayvon realized Zimmerman had a gun and began to fight to stop Zimmerman from pulling the gun from the holster, all while Trayvon pleaded for help, in desperate fear for his life.
Zimmerman ultimately won that struggle and feeling justified that the fight confirmed Zimmerman's suspicions about Trayvon, fired a single shot killing him , assuming the law would justify the shooting, as self defense because of the fight.
If it is Trayvon's voice on the 911 tape, Zimmerman should go to jail for life because, Trayvon fought to keep the gun from being pulled on him and pleaded for help for at least :50 sec in fear of his life. Trayvon's fears would be proved valid, the moment Zimmerman pulled the trigger.
Once Zimmerman had control of the weapon, he had control of the situation and even after hearing nearly one minute of cries for help, from a person he already identified as a teenager, he still pulled the trigger. An unrighteous kill.
If Zimmerman is on the tape, he should be acquitted as clearly his story of self defense is valid.