The different moral responses to Nazism and communism in the West can be interpreted as a result of the perception of communist atrocities as byproducts of noble intentions that were hard to realize without resorting to harsh measures. The Nazi outrages, by contrast, are perceived as unmitigated evil lacking in any lofty justification and unsupported by an attractive ideology. There is far more physical evidence and information about the Nazi mass murders, and Nazi methods of extermination were highly premeditated and repugnant, whereas many victims of communist systems died because of lethal living conditions in their places of detention. Most of the victims of communism were not killed by advanced industrial techniques.
Communist systems ranged from tiny Albania to gigantic China; from highly industrialized Eastern European countries to underdeveloped African ones. While divergent in many respects, they had in common a reliance on Marxism-Leninism as their source of legitimacy, the one-party system, control over the economy and media, and the presence of a huge political police force. They also shared an ostensible commitment to creating a morally superior human being -- the socialist or communist man.
Political violence under communism had an idealistic origin and a cleansing, purifying objective. Those persecuted and killed were defined as politically and morally corrupt and a danger to a superior social system. The Marxist doctrine of class struggle provided ideological support for mass murder. People were persecuted not for what they did but for belonging to social categories that made them suspect.