“Nazism and Marxism shared a desire to reshape humanity. ‘The alteration of men on a mass scale is necessary,’ wrote Marx; ‘the will to create mankind anew’ is the core of National Socialism, wrote Hitler. They also shared a revolutionary idealism and a tyrannical certainty in pursuit of this dream, with no patience for incremental reform or adjustments guided by the human consequences of their policies. This alone was a recipe for disaster. As Aleksandr Solzhenitsyn wrote in The Gulag Archipelago, ‘Macbeth’s self-justifications were feeble — and his conscience devoured him. Yes, even Iago was a little lamb too. The imagination and the spiritual strength of Shakespeare’s evildoers stopped short at a dozen corpses. Because they had no ideology’
The ideological connection between Marxist socialism and National Socialism is not fanciful. Hitler read Marx carefully while living in Munich in 1913, and may have picked up from him a fateful postulate that the two ideologies would share. It is the belief that history is a preordained succession of conflicts between groups of people and that improvement in the human condition can come only from the victory of one group over the others. For the Nazis the groups were races; for the Marxists they were classes. For the Nazis the conflict was Social Darwinism; for the Marxists, it was class struggle. For the Nazis the destined victors were the Aryans; for the Marxists, they were the proletariat. The ideologies, once implemented, led to atrocities in a few steps: struggle (often a euphemism for violence) is inevitable and beneficial; certain groups of people (the non-Aryan races or the bourgeoisie) are morally inferior; improvements in human welfare depend on their subjugation or elimination. Aside from supplying a direct justification for violent conflict, the ideology of intergroup struggle ignites a nasty feature of human social psychology: the tendency to divide people into in-groups and out-groups and to treat the out-groups as less than human. It doesn’t matter whether the groups are thought to be defined by their biology or by their history. Psychologists have found that they can create instant intergroup hostility by sorting people on just about any pretext, including the flip of a coin.”
From Steven Pinker’s The Blank Slate: The Modern Denial of Human Nature, one of the most illuminating, easy-to-read books about science and philosophy out there.