As people across the racial spectrum gathered Friday to mourn and condemn the deadliest attack on police in recent history, a group of fringe black extremist organizations found cause for celebration over the deaths of cops. After sniper fire brought down five officers and wounded more at a Black Lives Matter march in Dallas Thursday evening, the group issued a swift statement denouncing violence. Their statements were echoed by the president, political leaders from both parties, and across social media. But, some members of black vigilante hate groups seized on Thursday’s sniper shooting in Dallas to issue a rallying cry...