It might well have occured exactly as you described it, but there is no proof that it did. There is not a single piece of physical evidence that supports the theory that Zimmerman initiated the physical altercation. The only piece that even comes close to indicating something of that nature is the testimony of his girlfriend, who has absolutely no credibility becasue she has changed her story and told several documented lies. On the other side you have actual recordings of Zimmerman's phone call with the police when he states that he has lost sight of Martin and is returning to his truck to await the police.
Zimmerman is, beyond a doubt, an idiot, and apparently a complete wuss when it comes to fighting, but neither of those is a crime.
On a side note I found it rather amusing (or sad) how the prosecuter in his closing talked about (his assumptions of) Zimmerman's assumptions that night (kid was on drugs, a criminal, and appeared to be casing the homes for robery) and how they were wrong (when they were actually rather correct, Martin had THC in his system, and had been previously caught with burglery tools and stolen jewlry). Can anyone explain to me why the prosecuter is able to characterize Martin in this way, but the defense is not able to introduce rebuttal evidence?