The fatal shooting of 17-year-old Trayvon Martin last month by a neighborhood-watch volunteer was a sickening and — unless new facts come to light — unjustified loss of an innocent life. Unless Martin’s killer, George Zimmerman, had reason to believe that Martin was armed, shooting him was a grossly disproportionate response to a fistfight, even leaving aside the fact that Zimmerman had initiated the encounter. If such a shooting is justified under Florida’s broad self-defense law, that law has licensed violence that goes far beyond legitimate self-defense. Every American shares the despair of Martin’s family over this heartbreaking tragedy...