Marx was explicitly clear, the working classes should be the dictators of society when building towards communism. The USSR was run by the working classes, for the interests of the working classes. All states are inherently "authoritarian," as they are all monopolies on violence. Speaking as though a worker-state could somehow not be a monopoly on violence just eschews Marxist analysis entirely. Is your point that Marx was a "Stalinist?" This is absurd. The USSR was Marxist-Leninist, and "Stalinism" does not exist.