The dogmatic philosophical movements are often lacking in good manners because they see every disagreement and incorrect statement as a sign of a moral failing. Here’s a perspective from Eric Hoffer (from an entry in his diary on June 16, 1958):
Good manners are inconceivable without a degree of objectivity, and the give and take of compromise. He who clings with all his might to an absolute truth fears compromise more than the devil. He throttles the soft amenities which would dovetail him with others, and blur his uncompromising stance. Thus it happens that when a faith loses its potency rudeness often serves as a substitute.There is nothing worse than a philosophical movement that is motivated by petty moralism and expects total conformity with its ideas.