To what degree do the catastrophes and atrocities of the socialist/communist governments reflect on socialism and communism? A related question: to what degree do the catastrophes and atrocities of Islamic governments reflect on Islam? Yet another related question: Why do the catastrophes* and atrocities of capitalist/imperialist governments almost never reflect on capitalism? Yet another related question: Why do the the catastrophes and atrocities of science almost never reflect on science itself?
*It seems obvious that the massive urbanization of capitalist societies had a dramatic impact on the epidemiology of the Spanish Flu.
If some ideology, program or methodology — some some mode of thought — leads to severely bad results, should not those results be sufficient justification to abandon that mode of thought? Contrawise, if we are willing to preserve science and capitalism despite its catastrophes and atrocities, saying that they were not reflective of "true" capitalism and science, why then should we not preserve Islam and accept the excuse that the catastrophes and atrocities were not reflective of "true" Islam? And how does this distinction reflect on socialism?
It's simply insufficient to say, "If X leads to bad results, then X is bad." If we believed that, we would believe nothing else: every organized mode of thought, even Buddhism, has led to bad results. We must always — even in the case of Islam and Christianity — look deeper to understand and eliminate the actual cause of bad results. We must look deeper even into Islam and Christianity before dismissing them.
The key question to ask is: what is the core of the ideology, and what is the connection between the core ideology and the bad results? These are nontrivial questions.
In the case of Christianity and Islam, we can find the core ideology — or at least part of it — in its scriptures. Without some authority given in some sense to the Bible and the Koran you simply do not have Christianity or Islam at all. You cannot simply throw the Koran in the trash and honestly call yourself a Muslim; you cannot relegate the Bible to a work of entirely human literature and honestly call yourself a Christian.
If the scripture is part of the core ideology of a religion, then its canonical exegesis — its "interpretive schema" — must form the rest of the ideology. The interpretation must be canonical, endorsed by some authoritative body, to rise even to the level of ideology. Were there no canonical interpretation at all, then I — a dedicated "militant" atheist — could "honestly" call myself a Muslim: I certainly agree that the Koran exists, and if my interpretation is just as good as anyone else's then I'm just as good a Muslim as anyone else.
There's another problem too, a big problem, with religious scriptures. The plain meaning of the text is, in too many places, simply incompatible with humanistic ethics, ethics which place at the most foundational level the emotional and material well-being of human beings. The religious believer has a trilemma: abandon the authority of scripture, abandon the authority of humanism, or somehow interpret the anti-humanistic elements of scripture in a humanistic way. The first option leads to atheism; the second to "Phelpsism"; the third is simply schizophrenic, preserving humanism at the expense of rationality. And the third way is always incomplete. If you interpreted all the scripture humanistically you would just be, for all practical purposes, an atheist; why have a scripture at all if humanism is universally dominant? But the temptation to interpret scripture to justify one's prejudices is very strong: the inherent function of scripture is to justify ethical principles that cannot be justified humanistically.
So we see that fundamentally atrocities of religion cannot be divorced from the core ideology of the religion. Therefore we must conclude that the core ideology of religion is a sufficient causal reason for its atrocities, and rationally deserves the blame.
The core ideology of science is the scientific method. The scientific method is purely epistemic; it has almost no ethical content. Regardless of ethics, it is still true that if you compress a couple of kilograms of plutonium with some high explosives, you get a hell of a big bang. The only ethical content of the scientific method relates to our ethical beliefs about truth itself. Science tells us what the choices are, not — at a fundamental level — how to choose between them.
So in this sense, the core ideology of science is fundamentally good: To make choices, it is necessary and good to rationally apprehend the nature of the choices, good and bad. The core ideology of science can be divorced in an important sense from the atrocities of some scientists.
How about capitalism? It is clear that many (but not all) of the atrocities of capitalism, including imperialist world wars and wars of aggression, recessions and depressions, systemic poverty and hyper-exploitation stem directly from the notion of private property, especially absentee ownership. You simply cannot remove the atrocities without somehow compromising the notion of private property as an essential ethical principle, and you cannot have capitalism without private property any more than you can have Christianity without Jesus or Islam without Muhammad. Say what you will about Ayn Rand, but at least she's absolutely honest about this point: no atrocity, no catastrophe — not even the fall of civilization and the death of millions — justifies the least interference with private property. She was as vocal and dedicated an opponent of Keynesian liberalism as she was of totalitarian fascism, in just the same sense that one can and should be a vocal and dedicated opponent of the murder of one person as the murder of millions. And, of course, in just the same sense that if the Bible really is the word of God, one should be a vocal and dedicated opponent of throwing away (or interpreting away) a single verse as of throwing away the whole book.
What about socialism?
The problem with socialism (and communism) is that socialism has no core ideology. There is simply no widespread agreement over even a single point among self-identified socialists... especially if one counts the government of China, which still self-identifies as socialist*. There is no authoritative body whatsoever that specifies a canon of literature or a canonical interpretation. The word "socialism" — without additional qualification — actually means nothing at all. (Or almost nothing. If I tell you I'm a socialist, I've at least told you that I'm definitely not a Randian, that I do not consider private property an absolute, essential ethical principle. But even capitalists don't hold private property as absolutely inviolate. And I can more easily tell you that I'm not a Randian by saying that I'm not an idiot and a complete asshole.)
We simply cannot address socialism or communism as a category, good, bad or indifferent. We have to address individual expressions of socialism individually. If we want to understand the Soviet Union or Maoist China, we have to look at the Soviet Union and Maoist China. When we have learned all we can from them, we find we have learned about the Soviet Union's socialism and Maoist China's socialism (and sometimes just the Soviet Union and Maoist China), not socialism in general.
One persistent habit of scientists is that any general field of study is never named after an individual. It's evolutionary biology, not Darwinism; relativity, not Einsteinianism; electromagnetism, not Maxwellianism; quantum mechanics, not Heisenbergism or Schroedingerism. To name a field of study after person is to elevate their writings, if not to scripture, then to a core ideology, tying yourself to their errors and omissions as well as their insights. (The reference to specifically Newtonian mechanics is a reference to Newton's limitations, to make clear one is using a simplification and approximation of real (relativistic) physics.)
It's notable that the most pseudoscientific of the "scientific" disciplines, Freudianism, is indeed named after its founder. And don't even get me started on fucking academic philosophy, which is lousy with personalization.
For this reason, I have never and will never self-identify as a Marxist or Marxist-Leninist-Maoist. There is no doubt that Marx, Lenin, and Mao had profound insights, learned important lessons and explicated important ideas. If one believes that a revolution is necessary to overcome the evils inherent in capitalist/imperialist ideology, then it is foolish to ignore the only two people who have led successful revolutions against imperialist capitalism and attempted to actually implement a non-capitalist society. But on the other hand, I'm unwilling to allow even the writings of Marx himself to form my "core ideology"; to do so would be to tie myself to his failures as well as his successes. For the same reason, I will never join a group that explicitly calls itself Marxist-Leninist-Maoist, even if they are actually critical of Marx's, Lenin's and Mao's thought. For the same reason, if you want me take seriously what you have to say about psychology and psychoanalysis, you can't call yourself a Freudian.
I call myself a communist and socialist for the same reason (and with the same vagueness) that I call myself an atheist. Which is to say, I'm saying nothing except that I'm not religious, and I do not consider capitalism — even Keynesian regulated/stimulus capitalism — the bee's knees. Beyond that, if you want to find out what I do believe, you'll have to read what I personally have to say.