The pro-Marxist, such as Chomsky (who calls himself an “Aharchist” but his “Anarchism” is simply Marxian communism under another description) is incorrect in saying it “wasn’t really Marxism.” (Chomksy also tends to think that relatively small mobs in the streets, for example, the occupy movement, represent “the people” and “the workers,” even if they are only a tiny minority of the population. So in actuality, his expressions on this are not a matter of intellectual honesty/consistency, but simply a way to rationalize away past examples of implementing his preferred model).
Remember that under Marx’s thought, what little he said about the development of socialism & communism – one starts out with Socialism, a dictatorship. Blither to the contrary notwithstanding, this will always and everywhere be top-down, led by a “vanguard,” because under Marx’s own theory, people will not hold the correct views once the revolution comes; revolutionary terror (he is very explicit about this, and had only contempt for people who romanticized things) will and must be used to purge the population of people with wrong views, and indeed this is what “permanent revolution” was all about: “the killings just go on and on” as Thompson points out (link below).
Indeed another of the things Marx was explicit about was his believe that the problem with the French Revolution was that the leaders of it lost their nerve, that they didn’t engage in *sufficient* revolutionary terror. Marx’s critique had nothing to do with the hierarchical nature of the revolution as such.
As for the rest, I’ve linked to this FEE lecture by C. Bradley Thomson on Marx and Marxism, and the features within Marx’s theory itself that explicitly lead to precisely the sort of brutal outcomes that Marxism has always perpetrated.
Lenin, Stalin, Mao, Ho Chi Min – these were not people who somehow misunderstood Marxism, or used Marxist rehetoric illegitimately to impose a dictatorship. They were the best students of Marx, the best exemplars of what his structure demands in practice. Marxism is not some humane system, not some “good idea, it just hasn’t really been tried yet.” It’s not even some humane system that “it’s too bad it won’t work in practice.” It’s a horror show, a deliberate – conciously-outlined – horror show, and while Marx himself would have found people who claim otherwise to either be 1) usefully setting aside bourgeoisie morality regarding truth in order to persuade others rhetorically or 2) romantic dupes who themselves are useful for the revolution but have no true understanding of Marxism, the fact is he held in contempt romanticist communists who refused to admit to themselves the necessary measures that would have to be taken to pound humanity into the new form it would take in order to build communism.