The conviction of disgraced former Minneapolis police officer Derek Chauvin on both manslaughter and murder charges ought to be, at least in theory, a seminal moment for a country hoping to foster a greater civic unity and recoil from the tumultuous brink of a half-century nadir in racially charged acrimony. Sadly, early indications point toward something closely approximating the precise opposite. Successful criminal prosecutions of police officers for use of lethal force in the line of duty are rare in America, reflecting both the statistical paucity of truly egregious police misconduct and a still-widely held understanding that policing is extraordinarily...