The focus here is on Lenin and his progeny, particularly Stalin. Marx claimed the discovery of objective laws of reality through generous application of the Hegelian world-spirit concept. The Bolsheviks expanded Marx's ideas.
The expansion was thus: the Party is the agent of the Proletariat. The liberation of Proletariat is the ultimate end of History. The laws of History dictate that this is so. But if there are objective laws of history, someone must explain what those laws are and act on them: this is the Party. And if the Party observes such laws, then the concept of democracy or debate within the Party is nonsensical, so the Party must follow the doctrinal rulings of the Politburo. But it also follows that debate or democracy within the Politburo is also nonsensical, therefore, only one leader may give meaning, purpose and weight to the objective laws of history.
Leninism laid the ideological rails that Stalin used to railroad Russia straight to hell. Marx's work was not sufficient, but it was necessary.
 There is a view that Marx's ideas were purely economical in nature. I don't think this holds up. He was a philosopher first and an economist second. The concept of alienation for example is not just about effort or property, it is a quasi-mystical notion of a fundamental schism of persons, from themselves and from each other.