A sheriff’s deputy arrived at Nathan and Danielle Clark’s front door on the outskirts of Springfield, Ohio, in September with the latest memento of what their son’s death had become. “I’m sorry that I have to show you this,” she said and handed them a flier with a picture of Aiden, 11, smiling at the camera after his last baseball game. It was the same image the Clarks had chosen for his funeral program and then made into Christmas ornaments for his classmates, but this time the photograph was printed alongside threats and racial slurs. “Killed by a Haitian invader,” the flier read. “They didn’t care about Aiden. They don’t care about you. They are pieces of human trash that deserve not your sympathy, but utter scorn. Give it to them … and then some.” “They have no right to speak for him like this,” Danielle said. “It’s making me sick. There must be some way to stop it.” This was the version of the country the Clarks and their two teenage children had encountered during the last...