When people describe particularly evil individuals or regimes, why is it that they use the terms “Nazi” or “fascist,” but almost never “communist?” Given the unparalleled amount of human suffering communists have caused, why is “communist” so much less a term of revulsion than “Nazi?”
Communists killed 70 million people in China, more than 20 million people in the Soviet Union (not including about 5 million Ukrainians), and almost one out of every three Cambodians. And communists enslaved entire nations in Russia, Vietnam, China, Eastern Europe, North Korea, Cuba and much of Central Asia. They ruined the lives of well over a billion people. So, why doesn’t communism have the same terrible reputation as Nazism? -https://www.prageru.com/courses/history/why-isnt-communism-hated-nazism
Because although Nazisim, like other fascist regimes, professed hyper-nationalist authoritarianism which by definition resulted in oppression, war, and the mass extermination of an entire ethno-religious group; there is nothing intrinsically authoritarian about the writings of Marx, Engels, or any of the 19th century writers traditionally associated with Communism.
Although the individual actions of leaders acting under the pretext of fulfilling the promises of Communism are certainly authoritarian, the term is sufficiently broad that it can take multiple meanings; unlike Nazism, which is exclusively defined as the form of Fascism which emerged in Germany in the 1930s.
I actually recently wrote a very brief summary of Marxism in a thread yesterday. I also wrote about the Italian Communist Party's unique approach of Eurocommunism as well as the relationship of Italian Left-Wing parties to the Soviet Union, while in this answer I talk about the various Communist and Socialist-inspired currents of left wing thought and their influence in western politics.
You might be interested in this answer as well from awhile ago.