Korea. You mean like South Korea which had a military dictatorship at the time of the war? The problems with Korea largely come from a few issues.

1.) Strong USSR influence on their ideology of Juche. 2.) During the Korean war the United States bombed North Korea more than all the bombs dropped on Europe during WW2. We actually ran out of targets to bomb because we bombed so much. Still NK bounced back after the war and for awhile was much better off than SK was despite the horrific prison system there (which I won't even attempt to defend. But again, this is what happens when Russia is your primary influence. 3.) They are completely isolated from the rest of the world.

You also need to consider why Korea was ripe for revolution in the first place. They were colonized by imperial Japan and were enslaved and their entire way of life threatened by them. Many of the communists in the region started as liberation fighters attempting to remove the Japanese from their land. After the war America switched places with the Japanese and began to occupy Korea and installed a government friendly to them, a government filled with collaborators who worked with the Japanese during the occupation. Even before the Korean war broke out that pro American military dictatorship began executing people assumed to be communists slaughtering entire villages and committing horrible massacres. To many of the communists in the north they viewed the American imperial occupation no different than the Japanese.

China was extremely connected to and influenced by Stalin. Maoism, also called Marxist-Leninist-Maoist, or MLM is literally an extension of the Marxist Leninists beliefs created by Stalin and Lenin. Had their been a less authoritarian communist influence in the region who knows how Mao would have operated. There were certainly people who had libertarian communist beliefs in the region involved in the early resistance in both China and Korea to both European colonization and Japanese occupation. However these voices were largely silenced thanks to the strong influence of Stalin.

The Khmer Rouge has absolutely nothing to do with communism. Similar to China Korea and Vietnam, Cambodia was colonized by the Europeans, and Cambodian rebels were attempting to liberate their lands. Pol Pot was not a communist. He aligned himself with communism to get help from others but was not a believer. He literally wanted to destroy the working city dwelling classes and return cambodians to nature. That's the exact opposite of what communists believe. Not to mention that Pol Pot was supported by the United States

https://en.m.wikipedia.org/wiki/Allegations_of_United_States_support_for_the_Khmer_Rouge

As an attempt to weaken Soviet influence. And it was the communists in Vietnam who actually invaded and removed the Khmer Rouge from power. So if anything, Pol Pot is a strike against the west. Not against Communism.

All of your last paragraph is a critique of Marxist-Leninist belief and I agree that it is a terrible ideology. But just because ML is communist doesn't mean all communists are ML. As I pointed to before, the Spanish had a very strong libertarian communist influence in the form of the CNT-FAI who were defeated not because their government failed but because the Soviets played power games while the Fascists under Franco received troops and arms from Italy and Germany.

You also have the Makhnovists in Ukraine lead by Nestor Makhno who believed in the Soviet system (Soviet means workers council and not ML. The Soviet System is basically that the workers should get together and decide how to run things themselves). Makhno fought against both the USSR and the Europeans who were attempting to secure victory for the Tsars. Makhno liberated the villages from both the reds and the whites (USSR and tsarists respectively) and liberated the people. He told them that he came to destroy the old ways of power, taught them how to organize themselves, and said he didn't care how they did it, they just couldn't go back to how things were. Their movement grew and in 1919 they controlled most of Southern and Eastern Ukraine, including some major cities even though their base was primarily with the peasants. They instituted civil liberties in the region for the first time ever. Literacy rates skyrocketed, and the people successfully organized themselves into a non authoritarian Soviet system. The way that Lenin described how it should have worked before he came to power and switched up. They would have continued to work except they were betrayed. The Makhnovists aligned with the reds temporarily to drive out the last of the whites in the region and after a tough fight where the Makhnovists did most of the fighting they were ambushed by the reds the night of their victory while they were celebrating and drinking in their camp.

The Free Territory of Ukraine (the name the Makhnovists fought under) and the CNT-FAI both provide examples of how communism can and should work. Under their reign the people prospered and flourished. They were defeated by war and betrayal both. Not because their government didn't work. That would have been true even if they were liberal capitalists aligned with the West.

Today, while not explicitly communist (but definitely heavily influenced by communism) you have places like Rojava in Syria which is the single freest place in the middle east, attempting to bring Murray Bookchins communalism into being while creating an ethnic homeland for the Kurdish peoples. You also have the EZLN in Chiapas who are fusing leftist belief with traditional Mayan culture to create the last holdout of Mayan culture and civilization in Chiapas.

Neither the Kurds in Rojava, nor the Zapatistas in Chiapas are perfect. But they represent a resistance to imperial power and the hope of oppressed peoples to have their own homeland.

Communist beliefs are not authoritarian in nature. The ML's and MLM's mistakenly believe that you need authoritarian governments in order to destroy the capitalists and bring about communism which is supposedly their end goal. I do not support them in that. Communism can only come from destroying vertical hierarchies. You cannot bring about communism through the use of new vertical hierarchies, because as you've correctly pointed out it's too easy for someone to rise to the top. That is why the successful libertarian communist attempts first destroyed the hierarchies and organized the people along democratic horizontal structures.

If you are open to learning more and like to read I'd suggest you read The Conquest of Bread by Pyotr Kropotkin. It's a plan for how a communist revolution in Paris could have worked and organized itself. The first half is very interesting in my opinion. The second half gets bogged down in details and numbers specific to the ideas and reality of Paris in the early 20th century. But the vision Kropotkin lays out for society is amazing and one that I would absolutely love to experience.