“Since Rousseau and Kant,” writes Bertrand Russell in History of Western Philosophy, “there have been two schools of Liberalism, which may be distinguished as the hard-headed and the soft-hearted. The hard-headed developed through Bentham, Ricardo, and Marx, by logical stages into Stalin: the soft-hearted, by other logical stages, through Fichte, Byron, Carlyle, and Nietzsche, into Hitler…doctrines have developed, by steps that each seem natural, into their opposites.”
Russell was writing this during the Second World War before Hitler was dead and Stalin had done his worst.
He examines the difference between the two schools, which he labels as Continental (the soft-headed) and the British (the hard-hearted), although I note that two Britons are among the soft-hearted and only one among the hard-headed. Russell himself is among the hard-hearted.
The first difference is with methods. The British (John Locke, John Stuart Mill, and later Marx), he argues, accumulate many facts and draw modest conclusions. The Continental (Descartes, Leibniz, and later Nietzsche), in contrast, build a vast edifice of deduction from a pin-point of logical principal. The British have a solid pyramid based on facts, meaning that flaws do not bring the whole thing crashing down. The Continental school he compares to a upside-down pyramid balanced on a single point—and so prone to toppling over.
Stalin, building on Marx, was a man of facts and processes, whereas Hitler created a vast dream/myth based on the idea of the superiority of Aryans.
The second difference is in ethics. The British, following Locke, value pleasure and happiness, with Bentham arguing for the greatest happiness for the many. The Continental saw pleasure as ignoble, and opted for alternatives including power (Hobbes), being “noble” (Kant), strength (Nietzsche), and strong emotion/passion/heroism (Byron).
Stalin was perhaps trying to achieve the greatest happiness for the many, although killing millions along the way, whereas Hitler despised the weak and felt it a duty to destroy them in pursuit of power, strength, heroism, and nobility.