You cannot determine either the truth or falsity of communism from the behavior of Stalin or the U.S.S.R. In the same sense you cannot decide the truth or falsity of the divinity of Christ from the behavior of Pope Ratzo or the Catholic Church, as evil, predatory, oppressive and blatantly evil the Catholic Church might be. Give me a list of the Church's crimes ten thousand pages long (and I'm not at all certain I'm being hyperbolic), and this list by itself would be not constitute a shred of evidence against the divinity of Christ. The atheist argument against the Catholic Church goes the other way. We know separately from the Church's behavior that Jesus Christ is not divine (and might not even be an historical person), therefore the Church's self-described belief in that divinity is no defense against its evils.
In much the same sense, that Stalin called himself a communist is no defense against his own crimes. (I suspect Stalin's crimes have been greatly exaggerated by a hostile and openly biased Western press and intelligentsia, but the general assholiness of world leaders of every ideology throughout history is pretty well-established, so I'm willing to accept on general principles that Stalin was one asshole among many.) And, similarly, give me a list of Stalin's crimes ten thousand pages long and that list by itself would not constitute a shred of evidence against communism.
You have to make an independent definitional argument, or you have to make a causal argument. "Stalin was a communist; Stalin was evil; therefore communism is evil" is trivially fallacious (hint: the structure of the fallacy is identical if you substitute "was an atheist" or "wore a bushy mustache" for "was a communist"). Communism is most definitely not defined as "whatever Stalin did". In all my years of studying politics and economics, even before I was a communist, I've never seen a good historical argument making either the definitional or causal argument. Before I became a communist, my belief that communism was evil was simply received wisdom, unskeptically and uncritically accepted. (I dismissed the more obvious anti-communist bullshit propaganda, but I believed that this was just actual critical thought repackaged for the credulous masses; imagine my surprise when I found out that the "scholarly" critique against communism was equally bullshit.)
So, show me how either definitionally or causally the socialization of capital entails rounding up tens of millions of people (on some accounts literally a third of the population of the Soviet Union) and killing them. Until then, I'm just going to treat your magic invocation of "Stalin" as just that: credulous, unskeptical thinking.
What is communism?
What is socialization?
What is ownership?
What is capital?
(Or, if you're curious, type "socialize capital" in the search box at the top of the sidebar.)