The death of Socrates is the first recorded example of a cancel mob in action. The Cultural Revolution in China is probably the most well known example of puritan popular condemnation escalating to mass murder. Following a similar model to the Inquisition and the Salem witch trials. Although more recent examples include the Rwandan, Myanmar and Palestinian genocides.
We're lucky that recent puritan populism in the anglophone world hasn't usually resulted in murder. Although there are a handful of counterexamples, what's protected us for the most part is also democracy, but in the non-majoritarian sense of the word. Human rights protections, due process and jurisprudence, media diversity ("freedom of the press"), adversarial legal and political rituals into which antipathy towards The Other can be channeled, and so on.