When a theocracy or political movement dehumanizes its opponents to the point that “the ends justify the means”, there is no rational basis for civility.
Or as Robespierre highlighted during the French Revolution: “we understand that it is utterly useless to appeal to reason and morality in dealing with ideologues. For they are convinced that reason and morality are on their side and that their enemies are irrational and immoral simply because they are enemies.”