If a man happen to take it into his head to assassinate with his own hands, or with the sword of justice, those whom he calls heretics, that is, people who think, or perhaps only speak, differently upon a subject which neither party understands, he will be as much inclined to do this at one time as at another. Fanaticism never sleeps: it is never glutted: it is never stopped by philanthropy; for it makes a merit of trampling on philanthropy: it is never stopped by conscience; for it has pressed conscience into its service. Avarice, lust, and vengeance, have piety, benevolence, honour; fanaticism has nothing to oppose it.
Jeremy Bentham, Introduction to the Principles of Morals and Legislation
conscience, fanaticism