Marxism actually requires an almost obsessive degree of democracy at every level and a high degree of decentralisation. (Yes I can spot the potential weakness!) I don't think anyone is accusing Mao or Stalin of that. Marx and Engels were actually gentle people in their private lives. Engels, in particular, went to great trouble to educate himself about other people's needs. He himself came from a wealthy weaving family in Wuppertal. Stalin and Mao were thugs, who used whatever political clothes, which suited their purpose, to get themselves into power.
It works both ways. One of the darlings of the right in post war Germany was the Bavarian bully Franz Josef Strauß. It emerged after his death that he had earned a considerable part of his fortune by negotiating arms deals with the DDR. Did that make him a good "Communist" or a good "Conservative"?