Americans have gotten into the uneasy habit of wondering how low President Trump might go each day in his multiple tirades against those he imagines as enemies, critics and easy targets. But did he really have to pick on the storm-ravaged American citizens of Puerto Rico, of all defenseless people, and reveal such a cold-hearted lack of empathy for their plight?Mr. Trump’s salvo of tweets on Thursday as the island struggled with devastation — suggesting people were ingrates responsible for much of their suffering — set a new low, even meaner than his usual harangues and self-aggrandizement. It called into question whether Mr. Trump grasps and accepts the responsibilities of his office when he imperiously suggested that federal emergency aid and workers could not be maintained on the island “forever.”In his rant, Mr. Trump made it sound as if the island’s 3.4 million residents were at his executive mercy. But in fact the federal emergency process, backed by congressional funding, typically stations workers on site for years to help communities recover from natural disasters.