This is why I keep saying, even if you believe Zimmerman's story, he committed manslaughter. [View all]
An excerpt from Florida's self defense laws, via Wikipedia. Emphasis is mine.
776.041 Use of force by aggressor. —The justification described in the preceding sections of this chapter is not available to a person who:
(1) Is attempting to commit, committing, or escaping after the commission of, a forcible felony; or
(2) Initially provokes the use of force against himself or herself, unless:
(a) Such force is so great that the person reasonably believes that he or she is in imminent danger of death or great bodily harm and that he or she has exhausted every reasonable means to escape such danger other than the use of force which is likely to cause death or great bodily harm to the assailant; or
(b) In good faith, the person withdraws from physical contact with the assailant and indicates clearly to the assailant that he or she desires to withdraw and terminate the use of force, but the assailant continues or resumes the use of force.
In plain English, having provoked the confrontation with Martin, no matter what happened from that point on Zimmerman's only legal recourse was to attempt to retreat.