Not long ago, a Texas native named Shamsud-Din Jabbar pledged allegiance to the Islamic State group on Facebook. Now, perhaps if he had been an election-denying vaccine skeptic, Jabbar might have gotten himself noticed by law enforcement. Instead, Jabbar was free to ram his rented Ford pick-up truck into a crowd on Bourbon Street in New Orleans, murdering 15 people. Jabbar, who had guns and explosive devices in his truck, was killed in a shootout in which he wounded two police officers. Law enforcement officials and media, as is their wont, reflexively downplayed the Islamic aspects of mass murder. The...