You left out Stalin. Arguably the worst of the tyrants of the 20th century. I do not believe that he has any claim to being a communist.
OTOH, I don't think that genuine communism scales effectively. On an extremely small scale it's one of the most humane systems. A healthy family operates this way. Scaled up to a village, there needs to be a strong ideology backing it. Usually religious, but not always. Even so, the cracks start showing. Larger than a village and there is generally an increasing requirement for force to hold things in place.
Note that communism is not (necessarily) Marxism, and the most successful forms (lasting more than a decade) are NOT Marxism. Marxism wants to be applied at a large scale, where communism does not work without extreme force, and such an application of force tends to lead to tyrants of one stripe or another at the top. Lennin, I believe, was a genuine Communist. (Note the capital C...that denotes Marxist flavored communism.) I did need to use an increasing amount of force, because that's the nature of the beast, but he also used ideology, which reduced the need...though not enough. He did, however, create a situation that was ripe for a non-ideological tyrant to take over. And this was probably unavoidable. Communism of either flavor doesn't work on a large scale. (I don't know about Anarcho-Syndicalism. I have my doubts, but perhaps it could scale to a small country.)
Please note, it's not clear that Democracy (in the US style) is stable when scaled to a large country. It worked pretty well when the power was held by the states, but with the feds holding the power it seems to be rapidly devolving into a plutocratic tyranny. How long the plutocrats will hold power over the tyrant isn't clear, but in Rome it worked for a reasonable while before a tryant seized power. It did cause a few civil wars as the citizenry rebelled against the plutocrats, but the plutocrats won...so when the tyrant seized power, the citizenry didn't care, and were actually hopeful that things would improve. And they, sort of, did. The tyrants didn't oppress the common people as much as the plutocrats had. (Most of the violence was at the upper layers.) OTOH, the romans didn't have robots and didn't have a police force. ISTM that the development of robot soldiers is specifically aimed at making civil war only winnable by the government. Similar considerations may go to the distribution of military equipment to the local police forces. Also the establishment of police checkpoints at such places as the entrances to hospitals, airports, etc. (I was shocked the last time I went to the Emergency Room to find that a checkpoint had been established at the entrance.) Currently many of them seem to be more security theater than real, but such things can, once in place, be tightened at will.
So ISTM that Democracy is currently failing in the US, and steps are already being taken to win an expected civil war. (I'm not commenting on the farce that elections have become. That's an old story by now, except at the very local level.)