Feudalism and slavery were awful, but don’t come close to the concentrated human misery inflicted by the utopian projects of the 20th century.
By that I assume you're largely referring to the Soviet Union and China during the cultural revolution? Yeah sure, those were bad, but it's hard for me to say whether they were particularly better or worse than slavery (at least from the perspective of a slave).
Moreover, I think that the assumption that those 20th century projects failed because they were utopian is unsourced. I think I could equally validly claim that they failed because of a fundamental failure to understand basic economics and/or a deep lack of empathy or realism on the part of national leadership.
“Utopian” is used in this context to refer to efforts to rebuild society from the ground up on logical principles chosen in isolation.
The population of Ancient Egypt was about 1 million at the start of recorded history and about 5 million during the Roman conquest. Yet, even though the number of Egyptians in slavery were only in the millions at its highest it was crime that was very widespread in relation to the population.
1/3 of the world’s Jewish population was wiped out in a decade. At least 1/10th of the population of Ukraine starved in the two year Holodomor. Double-digit percentages of the entire populations of the Soviet Union, Poland, Greece, and Yugoslavia perished in the Second World War as did about 8% of all Germans. 1/4 of Cambodians died at the hands of the Khmer Rouge. All within brief periods of time.
Yes, but this is not about their eventual failure, but about their Dystopianism, which was part of their programme. Lenin explicitly stated that his "Jacobian party would never reject terror, nor could it do so". Lenin espoused state terror and mass murder, justified by "the greater good". If that is not idealism causing Dystopia right here, I don't know what is.
It’s fair to say that when communism was merely awful rather than utterly catastrophic, it was merely because at those specific times and places, it was still pursued with at least a nod to pragmatic concerns. It’s no coincidence that the best form of communism—Deng’s “Socialism with Chinese characteristics”—was the most explicitly pragmatic variation of communism.