In self-defense cases after a charge is brought up the burden of proof IS actually on the accused to prove self-defense.
Self-defense is an affirmative defense. Zimmerman would affirm he actually shot Trayvon Martin and that shot led to Martin's death. However his defense would be self-defense, thus if he's charged he has to prove that he was acting in self-defense. At least... this is how I understand it.
Uptown, with many self-defense statutes, that's how it works. This particular Florida law looks different, in that all Zimmerman has to show is that he acted lawfully, up until the moment he drew the gun and fired. That's a CRUCIAL distinction. With most old-style (common law) self defense statutes, Zimmerman would have to show that he (1) did not initiate the confrontation unreasonably, (2) that he did not further aggravate the situation unreasonably, (3) that his fear of death or serious bodily injury was reasonable, and (4) that he was unable to flee without unreasonably exposing himself to threat of death or serious bodily injury. The standard for "reasonable" is normally ordinary common sense. That's how the law we have here in SC works. Now it appears, that under the applicable Florida law, Zimmerman only has to show one of those things (number 3, above) and that he was where he was lawfully, and was not in violation of another criminal statute. Not only is that a lower burden of proof; on top of it, the "lawfully present" and not acting "unlawfully" language seems to put the burden of proving that part back on the state, as a person is normally PRESUMED to be acting lawfully, absent a showing of a violation of (another) statute. By that standard , an act may be stupid, or even reckless, without being "unlawful". That part of it may well be why the State Attorney who initially reviewed the case refused to charge it, and if I understand correctly that this is a fairly new statute, may also account for some confusion on the part of the Sanford police as to whether they thought Zimmerman's actions appeared to be self-defense. To put it another way, under the law in SC, he likely would NOT be able to make an affirmative showing of self defense; where under the Florida law, as written, the prosecution has a FAR more difficult case. I really think, from a lot of comments I've seen here, that not quite fully appreciating the difference has created considerable misunderstanding of why the case wasn't charged in the beginning, and why it's not a given that it will (or even should be) charged now, unless new evidence comes to light. To further complicate matters, the more recent the statute, the less guidance there is, in the way of established judicial precedent in interpreting it. This whole thing may be less about race, or recklessness, than about confusion over a law that looks extremely loose in its wording. Ever heard of making something so simple it becomes confusing and complicated? That may be the case here; not so much what the law says, as what it DOESN'T say.