I couldn't even tell you which of the current Communist states became so via democratic means, as you've stated there have been many of.
The current official Communist states are, IIRC, China, Cuba, Vietnam, and Laos. Not many left at all, huh? But you're still missing my point. All there have ever been (at least long-term) are Communist tyrannies (or, as someone else pointed out, socialist dictatorships that call themselves Communist, which is why I'm using a capital `C').
But I think that part of that can be explained by the fact that the rest of the world that was opposed to Communism made sure that no Communist ever came to power. The only times they failed were when the Communists came to power in a coup of one nature or another. It's hard to foment revolution against another revolution.
And, to be clear, the western world (I don't want to put the US in a bucket of it's own here, as it wasn't alone in these actions, although it played an important role) was opposed to the very existence of a Communist state, not to protect the citizens of that country, as pretty much every ruler that was put in power in place of a potentially Communist regime was awful, and they knew it at the time. (If you read memoirs and interviews of CIA people, et al., who put these people in power, you'll see the phrase ``We knew he was a thug at the time'', or something quite similar, a
lot.)
And, to defend my example of Cuba, I said at the beginning that there are significant human rights problems in Cuba that need to be solved. But the US has no interest in that. They refuse to recognize Cuba diplomatically, which is ridiculous; that government has been in power for, what, fifty years? Many people claim that Cuba's poverty is largely due to the embargoes that the US levies against it. I don't know that that's definitely the case, but it's not outside reason.
My belief is that the US should agree to recognize Cuba, lift embargoes, and establish normal trade relations (assuming the couple of requirements that entails for any country) in exchange for Cuba cleaning up its human rights problems. Then again, the same sort of thing was set up with China, and their human rights problems are still significant, even if they are better.