Donald Trump had by far his worst debate of the 2016 campaign on Thursday. He was defensive and vulgar, evasive and condescending, rude and imperious. He moved drastically to the center on immigration, repudiating his position on H-1B visas on stage and saying “everything is negotiable,” including the border wall and the fate of illegal immigrants already in the United States. He defended his calls for expansive torture and the killing of civilians related to terrorists, saying the military “will do what I tell them” even if his orders contradict the law. He dismissed his opponents as “little Marco” and...