In a Walmart parking lot in Portsmouth, Virginia, in 2015, a white police officer named Stephen Rankin shot and killed an unarmed, eighteen-­year-­old black man named William Chapman. “This is my second one,” he told a bystander seconds after firing the fatal shots, seemingly in reference to an incident four years earlier, when he had shot and killed another unarmed man, an immigrant from Kazakhstan. Rankin, a Navy veteran, had been arresting Chapman for shoplifting when, he claimed, Chapman charged him in a manner so threatening that he feared for his life, leaving him no option but to shoot…