776.032?Immunity from criminal prosecution and civil action for justifiable use of force.—
(1)?A person who uses force as permitted in s. 776.012, s. 776.013, or s. 776.031 is justified in using such force and is immune from criminal prosecution and civil action for the use of such force, unless the person against whom force was used is a law enforcement officer, as defined in s. 943.10(14), who was acting in the performance of his or her official duties and the officer identified himself or herself in accordance with any applicable law or the person using force knew or reasonably should have known that the person was a law enforcement officer. As used in this subsection, the term “criminal prosecution” includes arresting, detaining in custody, and charging or prosecuting the defendant.
(2)?A law enforcement agency may use standard procedures for investigating the use of force as described in subsection (1), but the agency may not arrest the person for using force unless it determines that there is probable cause that the force that was used was unlawful.
In most situations, an affirmative defense wouldn’t play into the probable-cause determination. In a Florida case in which the accused may have used force (including deadly force) in lawful defense of himself (776.012, 013) or of others (776.031), the agency may not arrest the person for using force unless it determines that there is probable cause that the force that was used was unlawful.
In light of that, the action of the Seminole County State Attorney’s Office in declining to charge George Zimmerman early in the investigation seems less unreasonable: the law doesn’t allow an arrest if Zimmerman was acting in self-defense.
Was Zimmerman acting in self-defense?
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.
If a jury believes (beyond a reasonable doubt) that Zimmerman initially provoked the use of force against himself, Zimmerman loses unless (the jury has a reasonable doubt about whether) Zimmerman didn’t have any other options to prevent great bodily harm to himself.
That’s some pretty fuzzy stuff there; you can understand why a small-county SA might decide that the better part of valor, when faced with a guy who has a plausible self-defense claim, is to let him go and let the police finish their investigation.
(Thanks to Miami criminal-defense lawyer Brian Tannebaum for directing me toward the appropriate portion of the Florida Statutes.)