The mass shooting inside a crowded, Buffalo, New York, supermarket on Saturday, which killed 10 people and injured three more, is renewing fierce condemnation of the racist conspiracy known as the “great replacement theory,” after a racist manifesto believed to have been written by the gunman was uncovered online. The theory is popular among white supremacists and is predicated on the racist falsehood that white people are purposely being replaced by people of color. It’s reportedly all over the 180-page manifesto written by the alleged gunman, a white 18-year-old who drove hours from his home to perpetrate the attack, in...