I haven't looked into Mao or Pol Pot. If those situations resulted in a minority ruling over the majority I don't think this is what Marx meant by "dictatorship of the proletariats".
No it was the dictatorship of the ignorant, jealous, grabbing majority that justified unlimited tyranny. And once the most wealthy and powerful are dead the bar gets lowered further until there is a race to the bottom; where poverty and ignorance are the greatest virtues. Where soft hands are enough to justify execution and possession of a scarf deserves hours of public humiliation.
Please do research The Khmer Rouge and nightmare of Mao tse-Tung's revolution.