Fair enough. The world has seen various "Marxist" regimes that were all variations on socialistic totalitarian states. The implementation of Marxism has always taken a local turn to fit the proclivities of the leaders that steers it away from Marx.
Most Western countries have had governments which were at least partly comprised of Marxists have successfully participated in democratic governments at a regional or national level in much of Asia and South America. Marxists lead a successful transition to democracy in Nepal, and governed the country for most of its post-transition period. The current president and prime minister of Nepal are both members of the Communist Party, having traded the office back and forth with the liberal Nepali Congress over that period.
The over-representation of authoritarian strongmen among "Marxist" regimes is not a character of Marxism so much as it is of post-colonial regimes in general, and reflects the popularity of Marxism-Leninism as a political program of anti-colonial political movements in the mid-to-late twentieth century. There are enough examples of non-Marxist authoritarian regimes in post-colonial countries- in Algeria, in Iraq, in Burma, in Iran- to bring this identification with Marxism and authoritarianism into pretty serious question.
The over-representation of authoritarian governments in post-colonial regimes is a reflection of the tendency for non-authoritarian regimes to be susceptible to Western interference, as in Chile, Indonesia or Iraq. "Kill your enemies before they kill you" was a lesson well-learned by many would-be anti-colonial revolutionaries studying at the feet of colonial overlords. You have cited China as an example of Marxist authoritarianism, but the immediate predecessor to the CCP was another authoritarian anti-colonial revolutionary government, and arguably one that was, in terms of blundering self-interest and of sheer political violence against political enemies, considerably
worse than the Communists, which itself succeeded a clique of military warlords who were
even worse, because China's brief experiments with democracy were brought to a close by Western-backed military strongmen also as soon as they had begun. This is not to defend authoritarian post-colonial regimes, but to clarify that they are historically so prevalent because they are only kind of post-colonial regimes which Western imperialist powers, in their fecklessly grasping short-sightedness, permitted to exist.