Not every single Communist country has come about from revolution. Cambodia became Communist through election of Communism-supporting leaders due to Vietnam starting a Communist revolution right next door.
And then what happened? Pol Pot took over peacefully, there was no revolution/war in Cambodia, yet Pol Pot ended up committing mass genocide against his own people. Marx wrote religion was the "opium of the masses" and could have no place in a utopia, because he wasn't very bright. So Pol Pot proceeded to torture many of the large Buddhist population in Cambodia, execute them, or turn temples into gulags/torture chambers. He then proceeded to commit a similar atrocity compared to Stalin slaughtering most of his high-ranking military officials. Pol Pot ended up executing most if not all of the left leaning government officials still left once he took power.
Many many more horrible acts were committed by Pol Pot, all because of him trying to enforce Communism on his country. Is Communism inherently bad? Not necessarily. The problem is that humans are just incapable of pulling it off correctly. It will never work because a classless system is sadly just impossible, and trying to make it so always results in 1 man having all the power. That 1 man then proceeds to go mad with power and kill everyone. Happened with Stalin, happened with Mao, happened with Pol Pot, happened with Castro, happened with Ho Chi Mihn, and so forth and so on.