Actually there's really not much difference between Communism & Fascism/Nazism.
Agreed that there is significant overlap, especially with regard to how they actually treat people, as is inevitable between totalitarian ideologies.
However, the differences are highly significant. The two most important are: 1. organizing principle; 2. attitude towards private property and the market.
1. Communism has as its organizing principle the grouping of the proletariat (by extension all lower classes) throughout the world in a giant coalition against their oppressors in the upper classes. Nazism/Fascism has as its organizing principle the coalition of all classes in the nation (Fascism) or "race" (Nazism) against all other nations or races. This is a huge difference, which by itself makes Nazism/Fascism ideologically incompatible with Communism.
2. Fascism/Nazism, in practice if not always in theory, allowed considerable scope to private property and the market. Communism by its very nature was forced to destroy the market, as the promotion of human economic equality was its very reason for being. That this issue is critical is seen by the repeated examples of communist governments destroying their own economies in the attempt to stamp out capitalism. Any "Communist" party that abandons this purpose is no longer Communist in any meaningful sense. OTOH, Fascism/Nazism, to the extent it was an issue at all, saw the destruction of the market as a very secondary goal, to be taken up only after the complete victory of the nation/race over all opposition.