I was going to write about about why Nazism is such a bad word while Communism, which has killed significantly more people, is by and large less derogatory than Nazism.
Most of the answer is that leftists who believe communism has never been faithfully implemented are deranged. The majority are able to see that Nazism was, and is, terrible. Communism is not charged with this kind of universal condemnation because so many still have dreams of a Communist utopia coming to fruition.
Can we just talk about how vain, and scary, that sentiment is; communism has never worked because it has never been fully implemented. Implying that the person saying this believes they, or someone they agree with, could do better. Can you also imagine the insanity that would ensue from someone making the same comment on Nazism?
Anyways, PragerU does a very good job of explaining the different treatment of the two failed ideologies.