In considering the Tryvon Martin case (where a black teenager armed with Skittles and iced tea was shot by a much bigger white guy on a "neighborhood watch", who later claimed "self-defense"), this timeline explainer from Mother Jones is indispensable if you want to understand the case and the context for these laws.  It is seriously messed up.  This is one example of how the law works:

Many readers have asked whether, given the 911 recordings, a case against Zimmerman would be easier than most homicides in which "self-defense" is cited by a defendant. In Florida, the answer probably is no: The courts' interpretation of the stand-your-ground law has been extremely broad—so broad that, to win an acquittal, a defendant doesn't even have to prove self-defense, only argue for it, while to win a conviction the prosecution has to prove that self-defense was impossible.

Numerous cases have set the precedent in Florida, with the courts arguing that the law "does not require defendant to prove self-defense to any standard measuring assurance of truth, exigency, near certainty, or even mere probability; defendant's only burden is to offer facts from which his resort to force could have been reasonable." When a defendant claims self-defense, "the State has the burden of proving beyond a reasonable doubt that the defendant did not act in self-defense." In other words the burden of proof beyond a reasonable doubt never shifts from the prosecution, so it's surprisingly easy to evade prosecution by claiming self-defense.

This has led to some stunning verdicts in the state. In Tallahassee in 2008, two rival gangs engaged in a neighborhood shootout, and a 15-year-old African American male was killed in the crossfire. The three defendants all either were acquitted or had their cases dismissed, because the defense successfully argued they were defending themselves under the "stand your ground" law. The state attorney in Tallahassee, Willie Meggs, was beside himself. "Basically this law has put us in the posture that our citizens can go out into the streets and have a gun fight and the dead person is buried and the survivor of the gun fight is immune from prosecution," he said at the time.

Wow.  All you have to do is claim self-defense?  Even if it makes no sense?