Marx, socialism and communism
Neither Marx nor Engels provided a blueprint for the socialist state. There could in their view be no such thing as a communist state since under communism, with no class-rule or management needed, there would be no state because no classes. Even the Soviet Union described and understood itself as socialist, not communist : remember the Union of Soviet Socialist Republics.
The dictatorship of the proletariat
Marx did envisage a transition period between capitalism and (fully entrenched) socialism which he labelled 'the dictatorship of the proletariat'. He is sketchy on the details of this period. But some points are clear enough. Marx first referred to the dictatorship of the proletariat in the articles which became The Class Struggles in France 1848-1850; and he. Marx is sketchy on the details of this period of transition. But some points are clear enough. He explicitly has in mind the dictatorship of a class. There; and there is also by the strongest suggestionimplication the idea that while there is a dictatorship of one class over other classes, within the dictatorial class (the proletariat) there could be democratic institutions. In other words : the dictatorship of one class over others was consistent with democratic government within the dictatorial class.
Both Marx and Engels rejected 'Caesarism', a workers's dictator. Engels wrote :
The action of the masses does not, by a long way, mean personal dictatorship; indeed, where the masses abdicate their will, they are already on the road ttoto become, from a revolutionary factor, a reactionary one. (Ferdinand Lasalle as a Social Reformer, quoted in Hal Draper, Karl Marx's Theory of Revolution, III, The 'Dictatorship of the Proletariat', New York : Monthly Review Press, 1986 : 100.)
Marx and Stalin
It appears to me that Stalin was a 'workers' dictator' who practised 'Caesarism' to a degree that Caesar could barely have dreamed of. It's possible and even plausible to argue that the risk of Caesarism is inherent in any dictatorship, proletarian or other. To the extent that Marx discounted this risk under the dictatorship of the proletariat, he showed himself as 'utopian' as the socialists he had mocked so bitterly in The Communist Manifesto (1848).
But it is not special pleading or any kind of fallacy to claim that Stalinism was a distortion of anything Marx envisaged.