This statement actually points out how the discussion is mixing terms to create an non-equal comparison.
It is not legitimate to make a direct comparison between the Nazis and the Marxists as political or social strategies. If you wish to compare movements, the legitimate comparison is between Marxism and Fascism. (I would not hold up either movement as a shining example of humanity and will not defend their goals or their methods.) Cambodia is no more a natural and obvious result of Marxism than the Nazis were necessarily a natural outcome of Fascism. Each took ideas that were peripheral to the core beliefs and expanded them in ways that rationlized their killings. Nothing in Das Kapital provides for the elimination of anyone who is educated. Nothing in the speeches of Mussolini provides for the destruction of other peoples.
The Marxists tended to have a higher death toll than the Fascists because they tended to get hold of larger countries with more people. And, it should be noted, the great periods of murder under both the Fascists and the Marxists were generally carried out at particular times under particular leaders. The U.S.S.R. continued for forty years after Stalin without continuing his policies of genocide (which even he had cut back, over fifteen years earlier, once his political goals had been reached). Once Franco and Salazar achieved power, (in typically bloody ways), they did not carry on continued policies of murder. Similarly, once Tito and Castro had bloodily secured their control of their countries, they did not carry out repeated or continuous campaigns of terror or murder against their citizens. In both cases, the might of the government was enough to keep the citizenry in line without continuous blood baths.
The Nazis get the reputation for most evil because, unlike any of the other groups, they set out to murder people for the sheer sake of committing mayhem. The peoples who were systematically murdered by the Nazis were not a threat to their political goals and could have been incorporated into a Fascist system had they not been targeted for death. Jews, Rom, homosexuals, and the mentally incompetent all continued to live in Italy, Spain, and Portugal under the Fascist systems without threatening those systems in any way.
While the death tolls are much higher, the massacres committed in the U.S.S.R., China, and Cambodia were carried out to eliminate political opposition. This hardly justifies the atrocities, but it changes the motives under which people (and regimes) are generally judged.
(Of course, it is always more dangerous to be a citizen of a totalitarian regime, while it is generally safer to be a citizen of a more “free” nation, in which case there is more danger in being the citizen of a land on which a “free” country has designs.)