I’ve often wondered about that question myself: To what extent is repression inherent in Communism itself, and to what extent is it the product of accidental circumstances (Russian culture, Stalin’s personality, etc.)? As a student of history, I’ve come to the conclusion that it’s a bit of both.
Neither Marx nor Engels wrote very much advocating a repressive society, and both wrote quite a bit in the opposite direction. Lenin did institute considerable repression after the Bolsheviks took power (the “Red Terror”, Cheka, etc.), but it occurred in the context of a civil war and foreign invasion. When the threat ceased, Lenin tried to ease up (eg. the NEP), but he died before he could accomplish very much. Then came Stalin, and Communism was irreversibly shaped by what that murderous despot did.
All Communist regimes have been repressive to one degree or another, but the degree has varied considerably.
At one extreme, there was Hungary under Janos Kadar during the period from (roughly) 1963-1980, when citizens were free to travel to the West (most returned), and were free to read Western newspapers and magazines (except for a few strongly anti-communist ones). In Hungary during that period, there was actually quite a bit of private enterprise, and central planning was confined to setting very broad goals. It was still a repressive society by Western standards, but it was quite a bit freer than any other Communist country (except, briefly, Czechoslovakia) and even some countries in the “free world”.
At the other extreme, there have been Communist countries such as North Korea, Albania, and Cambodia under Pol Pot, where there was almost no personal freedom whatever.
One might also point out that the Hungarian Uprising of 1956 and the “Prague Spring” of 1968 were both led by Communists (Imre Nagy and Alexander Dubcek, respectively). So it would appear that Marxism-Leninism is susceptible to a variety of different interpretations, some more repressive than others.
But I think that there is a certain element of repression implicit in the Communist ideal itself. One of the flaws of a “command economy” is that it does not accommodate itself very well to human spontaneity. I’m no economist, but it seems reasonable to suppose that those in charge of such an economy will try to limit spontaneous human activities by adopting repressive policies. But, as I indicated above, the extent of both the central planning and the repression has varied quite considerably in practice.