At their convention five years ago, President Donald Trump and his Republican Party rallied their supporters fervently against an idea they characterized as a rot on society: cancel culture. Too many people, they argued one by one in prime-time speeches, were being publicly ostracized — in some cases losing their jobs — for exercising their constitutional right to free speech. “To the voiceless, shamed, censored and canceled, my father will fight for you,” Eric Trump pledged then. But the assassination of conservative activist Charlie Kirk, who was among the speakers at that 2020 convention, seems to have rapidly shifted how...