Why did communism always turn into a kind of dictatorship?
Afaik this happened with every single instance of a communist country. Communism seems like a pretty good idea on the surface, but then why does it always become autocratic?
Centralization of decision-making. It's ironic actually.
One of the main problems of capitalism that Marx described is the separation between labor and ownership. All the talk about "means of production".
Communism actually makes it worse. In capitalism yes you have the owners who have all the control and reap all the benefits, but you have many capitalists competing, so the power is kinda distributed inside the capitalist class. The way communism was always implemented is through a communist party and state control of the economy.
You get an even smaller group of people controlling the means of production. It amplifies exactly the main problem of capitalism by creating a very hierarchical class society where the party leadership takes a role of what is almost "nobility".
There's also just a fundamental problem with planned economies from a purely economic standpoint: they are much less efficient at actually providing the minimum set of goods and services required by a population, and they're worse at achieving growth. See the most recent Nobel Prize in economics for a citation. Funnily enough, the same paper's arguments apply equally to oligarchic economies and crony capitalist economies, which are semi-planned economies by a small group of the ultra wealthy.
More specifically to the OP, communist countries have planned economies, which by nature requires a strong authority to tightly control production. Hence why communist states always have very consolidated political power structures. And once the power is consolidated, all it takes is one bad actor to get that power and ruin everything.
Géza Hofi was one of the greatest comedians in Hungarian history. He was active under and very outspoken about the failures of the ruling communist party. One of his most memorable performances was "How many pigs will be born?" (video, unfortunately without subtitles).
Party officials, wearing nice brown trench coats, visit old man Joe's farm.
"Comrade Joseph, how many pigs will be born?"
"I don't know."
"Shut your mouth, peasant, and give me the number."
"What's the plan?"
"14."
"Then it'll be 14. Have you told the swine? Better that you talk to her, since you're both on the same level."
(the story goes on, but I don't want to translate the entire thing)
but you have many capitalists competing, so the power is kinda distributed inside the capitalist class.
This isn't always true, and is arguably not the natural state of capitalism. Capitalism, without state intervention, will tend towards monopoly as economies of scale and market power push out any competition.
Iirc this is what Trade Syndicalism was meant to solve. After all the talk about the people's rebellion it gets into balancing power by keeping it distributed among unions. So your political career would be to get elected in your union and then serve on the councils at different levels.