Gov. Jay Nixon of Missouri said on Tuesday that he would triple the number of National Guard troops in this suburban St. Louis city and broadly expand their role in keeping the peace, after a night of arson, looting and rampaging demonstrators showed that weeks of preparation for a grand jury decision in the fatal shooting of Michael Brown had failed to prevent violence. In Washington, St. Louis and Ferguson itself, an array of public officials, community leaders and clergy were deeply critical of one another as they sought to explain how protests over the grand jury’s decision not to...