Post-war Yugoslavia was such an example. As for the brutality, here’s a link: https://en.wikipedia.org/wiki/Goli_Otok#Prison
If an internal actor wants more power and starts making moves, you’d have to have consensus and collective action from the rest of the group to stop them. That is both very difficult and very expensive to achieve.
In practice, it turns into authoritarianism very different from the idealized goal.
> A communist society would entail the absence of private property and social classes, and ultimately money and the state (or nation state).[1]
Then starting from that, what the military does best is producing weapons.
That's the only country which wasn't at war which was proportionally spending that much on the military.
The Soviet Union was a military state disguised as a country, pretty much nothing outside the military really mattered there.
It's even visible on the inside organisation of the Soviet Union itself (which they picked themselves, nobody asked them), which is pretty much the same as in any army, just at a country level.
Sure, the US exploited that flaw but it's been there by design.
On the other hand, that excuse runs pretty thin after 1945. But the oppression was still there. So it appears that oppression was more a feature of the system. (Or at least a feature of the man - but then Lenin did it too, and Brezhnev did it perhaps less, but still enough to be oppressive, so it looks like it was a feature of the system.)
And the North's military so easily overwhelmed the South's that within two months, they were driven to brink of defeat, retaining only the Pusan Perimeter on the south-eastern tip of the peninsula.