Utopian socialism's failures are the reason Marxism came about as a structured, disciplined critique of class society. Idealism is no way to make people's lives better. Instead, you must understand the root causes of oppression in order to attempt to overcome it.

disciplined Marxist society also needs a manager class, turns out that managers have their own self interests - namely they needed stability/lack of purges and had the urge of turning public property into their own private property. Meet the new boss, same as the old boss.

OK, you're saying that capitalism is the problem, not the solution. But saying that disciplined Marxism is the solution hasn't worked out very well in practice, either - not better than capitalism, in fact.

None of those regimes claimed to be Marxist. Marxist-Lenist, Maoist, etc., sure.

Specifically, all of those regimes supported some form of "democratic centralism" and other aspects of Leninist party theory, such as the idea of a vanguard revolutionary party, which is not part of classical Marxism.

Amongst he first people purged by the Bolsheviks, were Marxists and other socialists that opposed their coup, and the resulting authoritarianism.

People who had spent months since the actual revolution, where power was handed to the Duma, building towards a liberal socialist society, and who won the vast majority of votes for the constituent assembly on that platform (the Bolsheviks won ~10% of the vote; hence the "October revolution", which was nothing more than a coup)

Many of them met their end for continuing to oppose the Bolsheviks politically, many after taking up arms and fighting on the other side in the ensuing Russian civil war. Some managed to flee.

I've tried to read Utopia but that book is written so dreadfully I had to drop it after 20 pages or so.

That would be the translator's fault (unless you can read Renaissance Latin -- which is not quite the the same as classical Latin).