Self defense is an affirmative defense in Florida. Zimmerman acknowledges he killed Martin, but he alleges this was justifiable in defense of his own life. So, his justification for the homicide must be shown by the defense.
If the jury believes, beyond a reasonable doubt, that Zimmerman started the altercation, his claim of self defense would not apply because he was the aggressor.
If the fact he needlessly followed Martin is seen as the primary cause of the altercation, he may be found guilty of negligent homicide.
I'm not at all sure Zimmerman will walk. I don't think they will prove second degree murder, but it's difficult to absolve Zimmerman of any responsibility for Martin's death since Zimmerman's actions were instrumental in creating the conditions that led to that death. I think a manslaughter conviction is a possibility.
And Zimmerman's injuries were nowhere near life-threatening, and Martin was unarmed, so a case can be made that he responded with excessive force by using deadly force.
I'm not willing to make any bets yet. They haven't even gotten to all the inconsistencies and contradictions in Zimmerman's accounts to the police. And some of what he alleges happened is not supported by DNA evidence.
I also found the testimony of the young woman who was on the phone with Martin credible--and, from her account, Martin was trying to get away from Zimmerman, not getting ready to attack him.
There are still too many unanswered questions, in my mind, regarding justifiable self defense.