Nearly a month after an unarmed black teenager was shot to death by a neighborhood watch captain, police in Sanford, Fla., have yet to make an arrest. Seventeen-year-old Trayvon Martin was returning from a convenience store near the house of his father's fiancee in a gated community Feb. 26 when watch leader George Zimmerman spotted him and called police to report a "suspicious" person. Moments later, Mr. Zimmerman confronted the teen — ignoring the dispatcher's advice not to follow the youngster — and shot him in the chest, apparently as Mr. Martin pleaded for his life. When police arrived on the scene, they found Mr. Martin had been carrying only a bag of Skittles and a can of iced tea. Yet they did not arrest Mr. Zimmerman, because they accepted his claim that he had acted in self-defense. Under Florida's Stand Your Ground Law, adopted in 2005, citizens can lawfully use lethal force to defend themselves if they have a reasonable fear of being killed or seriously injured. The case has sparked protests around the country over the authorities' refusal to charge Mr. Zimmerman. Mr. Martin's family has compared his death to that of Emmett Till, the 14-year African-American boy whose 1955 murder by Mississippi racists shocked the nation; others have condemned the incident as an example of the negative racial stereotyping that routinely punishes African-Americans for the crime of "walking while black." The week, the Justice Department announced it would open an investigation into whether Mr. Martin's killing violated federal laws. But it may not be able to bring charges unless prosecutors can show that the shooting was motivated by racial bias, and a spokesman for the department says that may be hard to prove. So far there's no clear evidence that the 28-year-old Mr. Zimmerman, whose mother is Hispanic and whose father is white, acted out of racial enmity. Yet, apart from the issue of race, the case illustrates the folly of Florida's Stand Your Ground law, which overturned a long-standing legal requirement that people who feel themselves in danger attempt to retreat before resorting to deadly force. Sixteen other states now have similar laws on the books, and the result has been a spike in homicides where perpetrators claim self-defense and their victims are no longer around to contradict them. The statute has become the favorite defense of Florida's drug dealers and gang members. It's a virtual license to kill. How can it be lawful for someone to hunt down an unarmed teenager and shoot him, then claim to have acted in self-defense? That's insane, but perhaps not much less crazy than the laws that allowed Mr. Zimmerman to carry a gun while he tooled around the neighborhood looking for "suspicious" characters. He was a loose cannon who took the law into his own hands, and his particular kind of lunacy will continue to happen unless the law is changed. Mr. Martin's family is right to demand justice. Regardless of whether this turns out to have been a hate crime, a promising young man lies dead, and his killer must be held to account.