Four years ago, when — as now — the nation was reeling from the horror of a mass school shooting, a retired Supreme Court justice suggested a radical solution: getting rid of the Second Amendment. John Paul Stevens issued the call after 17 people were killed at Marjory Stoneman Douglas High School in Parkland, Fla., in February 2018. The attack prompted hundreds of thousands to demand action the next month to end gun violence at the March for Our Lives. In a March 27, 2018, New York Times op-ed, Stevens praised the protesters and their call for stricter gun control...