There’s no such thing as a lone wolf — an appellation often given, in error, to terrorists who act alone, particularly those of the white supremacist variety. There are only those people who, fed a steady diet of violent propaganda and stochastic terror, take annihilatory rhetoric to its logical conclusion. Such was the case on Saturday, when a teenaged white supremacist named Payton Gendron opened fire in a supermarket in a Black neighborhood in Buffalo , New York, killing 10 people, while livestreaming the carnage on the live-video site Twitch. Prior to the shooting, he had posted a 180-page manifesto...