>the Nazis carried out fascism to the letter

Is there fascist literature detailing mass extermination? It seems they went beyond the letter.

And when speaking of communism, don't forget China which is also strongly authoritarian. If every implementation of Communism ends in tyranny then perhaps the ideology isn't compatible with reality.

I think that it's arguable that the Khmer Rouge carried out the closest attempt at pure Communism, but it's not like that turned out all that well.

That said, plenty of people associated things like the Stazi/KGB with Communism. I haven't actually read Marx's manifesto, but I highly doubt that his "worker's paradise" involved disappearing masses of people and a "report your neighbour, Comrade" mentality[1].

[1] Though I do know that he thought that there would be a progression like "Democracy => Socialism => Communism" and that each transition would involve bloody revolution.