The two teenagers who walked into a San Diego mosque with assault rifles on Monday evening wore patches displaying the Black Sun—a neo-Nazi iteration of the swastika—and had scribbled white-supremacist symbols in white correction fluid on their guns. They started shooting, killing three. Then they fled in a BMW one had stolen from his mother. In the car, 17-year-old Cain Clark apparently shot his accomplice, Caleb Vasquez, before shooting himself in the head. We know much of this, in graphic detail, because, within hours, Clark and Vasquez’s video-recorded rampage seems to have been posted on the messaging platform Discord, then on a website called Watch People Die. The tragedy at the Islamic Center of San Diego in many ways followed an all-too-common script. With horrifying regularity, a young man carries out a mass shooting with weapons bearing neo-Nazi or hateful references scrawled in white. The shooter typically wears paraphernalia designed to promote accelerationism: the concept that only the collapse of society can usher in an Aryan utopia. There may also be a manifesto pulling from …