The first few minutes of President Trump’s Oval Office address after the assassination of Charlie Kirk last week followed the conventional presidential playbook. He praised the victim, asked God to watch over his family and talked mournfully of “a dark moment for America.” Then he tossed the playbook aside, angrily blaming the murder on the American left and vowing revenge. That was stark even for some viewers who might normally be sympathetic. When Mr. Trump appeared later on Fox News, a host noted that there were “radicals on the right,” just as there were “radicals on the left,” and asked,...