He accused environmentalists of wanting to “kill all the cows.” He personally insulted the Muslim mayor of London. He questioned whether the United Nations should even exist. “I’m really good at this stuff,” Mr. Trump said. “Your countries are going to hell.”In his remarks, Mr. Trump lectured the United Nations and other countries about how they were failing, and aired a list of grievances. Soon after his speech, Mr. Trump made news in another forum.