Ben Brody says his life was going fine. He had just finished college, stayed out of trouble, and was prepping for law school. Then, seemingly out of nowhere, Elon Musk used his considerable social media clout to amplify an online mob’s misguided rants accusing the 22-year-old from California of being an undercover agent in a neo-Nazi group. The claim, Brody told CNN, was as bizarre as it was baseless. But the fact he bore a vague resemblance to a person allegedly in the group, that he was Jewish, and, that he once stated in a college fraternity profile posted online...