Critics of Florida's self-defense law object to its recognition of a right to "stand your ground" in public places, which eliminated the duty to retreat from an assailant. Yet many of these critics seem to believe they have a duty to stand their ground and never retreat, using George Zimmerman's shooting of Trayvon Martin as a weapon to attack Florida's law, no matter what the evidence shows.

The emphasis on the right to stand your ground is puzzling in the context of the Martin case, since Zimmerman's defense does not seem to rely on it. The 28-year-old neighborhood watch volunteer, who was released on bail this week after being charged with second-degree murder in connection with the February 26 shooting in Sanford, told police the unarmed teenager knocked him down with a punch to the face and pinned him to the ground, repeatedly smacking his head against the pavement. By Zimmerman's account, then, he had no opportunity to retreat.

Florida's law also has been blamed for delaying Zimmerman's arrest, and it did require that police have probable cause to believe the shooting was unlawful. But this is the same standard that applies to arrests for all other crimes, and whatever obstacle it may have posed proved temporary.

One unusual aspect of Florida's law that will be apparent in this case is that Zimmerman has a right to pretrial hearing at which he can try to convince Judge Kenneth Lester, by "a preponderance of the evidence," that he acted in self-defense. If he can meet that standard of proof, which requires showing it is more likely than not that his use of force was appropriate, the charge against him will be dismissed. But even if he went to trial, he would be (or at least should be) acquitted with that much evidence in his favor, since the prosecution has to prove beyond a reasonable doubt that he was not acting in self-defense—which, as Northern Kentucky University law professor Michael J.Z. Mannheimer has pointed out, would be true "in virtually every state."

Zimmerman's defense under Florida's law is that he was attacked and "reasonably believe[d]" shooting Martin was "necessary to prevent imminent death or great bodily harm." Contrary to New York Mayor Michael Bloomberg, who is leading a national campaign against Florida-style self-defense laws, that does not mean people "make their own decisions as to whether someone is threatening or not" and therefore have "a license to murder." The threat assessment has to be reasonable, meaning someone who uses deadly force has to show it was justified by the circumstances.

Even if the Trayvon Martin case does not really illustrate the shortcomings of Florida's law, it is possible that eliminating the duty to retreat in public places, combined with reinforcing the "castle doctrine" (which applies to home invasions) and extending it to vehicles, has encouraged avoidable escalations of violence. The law's opponents note that the annual number of justifiable homicides in Florida (excluding police shootings) nearly tripled after the law was passed in 2005, from an average of 12 between 2000 and 2004 to an average of 35 between 2006 and 2010.

Still, you would expect to see an increase in homicides deemed to be justified even if the law were working exactly as intended. The crucial question in assessing the law's impact, which the task force appointed last week by Gov. Rick Scott presumably will ask, is whether these homicides should be deemed justified.

In the meantime, it is worth noting that Florida's violent crime rate, which fell by 12 percent in the five years before the "stand your ground" law was enacted, fell by 23 percent in the five years afterward. Since 1987, when Florida adopted a nondiscretionary carry permit law that the Brady Campaign to Prevent Gun Violence blames for "year after year of carnage," the state's violent crime rate has been cut nearly in half.

Jacob Sullum is a senior editor at Reason and a nationally syndicated columnist. Follow him on Twitter.