Stalin was a dictator - which is antithetical to the core principles of communism. You cannot be both a communist and a dictator, they are diametrically opposed.

It's a shame Lenin died right after the establishment of the USSR, he disdained everything Stalin became.

Communism is and isn't authoritarian. The establishment is, and the struggle to prevent regress into capitalism is, but for the people who aren't trying to exploit others, it isn't.

Authoritarianism ≠ fascism

Authoritarianism + ultranationalism = fascism.

Communism isn't anything like fascism. Communism, by definition, rejects nationalism, and considers every proletariat family, regardless of nationality and ethnicity. The only enemy of communism is the bourgeoisie (dictators and fascists foremost)