Under the proposed change, prosecutors would essentially have to try a case twice, at a hearing and then at the trial, while making it easier for defendants to claim a right to shoot first (or stab or club or otherwise attack someone) and argue against prosecution on the basis of their fears.

The defects of the Stand Your Ground law, which was enacted in 2005, were highlighted in the case of Trayvon Martin, an unarmed black teenager, who was shot to death in 2012 by George Zimmerman, a self-appointed neighborhood watchman. Amid national furor, Mr. Zimmerman was acquitted after claiming self-defense at a trial in which the Stand Your Ground law was included in jury instructions.

More than 20 states have enacted similar laws. In Florida it has sparked a boom in self-defense claims that have left courts, prosecutors and the police wrestling with contradictory interpretations. Stand Your Ground immunity was quickly invoked by killers in barroom brawls and gang wars, according to a detailed 2012 study of nearly 200 cases by The Tampa Bay Times. Almost 7 in 10 people making the claim were not charged, while in 68 percent of the cases the victims were unarmed.

The Wild West mayhem invited by the law is currently the focus in a Florida courtroom, where a retired policeman is on trial for shooting a man to death in a movie theater over the use of a cellphone during the previews. Witnesses saw popcorn thrown at the shooter during the dispute, then heard a gunshot. In a pretrial hearing, the defendant insisted the victim had been “ready to attack.” The shooter said, “I would have to take decisive action if I wanted to survive this thing.” A ruling is pending.

Research by legal institutions, including the American Bar Association, found that the Stand Your Ground law, far from advancing public safety, has been followed by an increase in homicides, diminished victims’ rights and heightened racial injustice in enforcement.