There's no doubt that the communist revolutions in the USSR and PRC entailed some bad shit happening. Communism really isn't some hippy-dippy utopia, it's a real political-economic system that has to work in the messy world of real people, many of whom are evil and more of whom are at best ignorant and at worst stupid.
Many communists really do wish we had a time machine, and we can go back and tell Stalin or Mao: Don't do this or that: you'll be more effective as communists if you don't. Communists admire (or at least tolerate) Stalin or Mao not because of their authoritarianism, but because they used their authority to do their best to implement socialism. I don't know anyone who thinks that real authoritarianism (and the unfortunate jargon "dictatorship of the proletariat" does not endorse authoritarianism) is necessary to implement socialism or communism. A socialist society without a democratic mandate from the masses is doomed to immediate failure. Stalin and Mao's authoritarianism — to the extent that it really existed and is not an artifact of hyperbolic slanderous propaganda — is not an inherent feature of socialism; it's an artifact of the contingent historical circumstances of the Russian and Chinese societies in which socialism was first tried.
Be that as it may, it should be blatantly obvious to anyone that a communist revolution even in an advanced industrial country such as the US would still entail some bad shit. At the very least, even if a communist government were constitutionally elected and the requisite Constitutional amendments enacted, we can be confident that the Christian fascists — who are presently armed to the teeth — would rise up in revolt. Protecting the government would require fighting a civil war. And there has never been any war conducted anywhere under any circumstances that has not caused massive civilian suffering and casualties, and has not seen war crimes and crimes against humanity perpetrated by both sides. Furthermore, even if a communist government were to win that civil war, it would probably have to repress Christian fascist ideology — indeed it's arguable that it would be irresponsible not to do so.
Radically changing the economic foundations of a three-hundred million person country is going to cause profound dislocations, suffering and some deaths. The government will make mistakes, serious mistakes, and those mistakes will also cause real suffering and death.
In short even the best case of a communist "revolution" would entail that some bad shit would go down.
It's every individual's personal moral decision to weigh the costs of a revolution against the benefits. We've developed a lot of rationalizations ("They started it!") but we're fundamentally teleological beings, capable of predicting the consequences of our actions. We cannot escape responsibility for those consequences however well-intentioned our actions. But it's equally the case that we have some similar responsibility for not acting, precisely to the extent that we can foresee the consequences of passivity.
It's entirely legitimate — indeed necessary — to honestly and realistically evaluate the pros and cons of a communist revolution — and there will be, without a doubt, cons. And I can see how a well-intentioned, thoughtful person might say after such an evaluation that revolution just isn't worth it.
There are other criticisms, though, that are as irritating and intellectually vacuous as the criticism that all atheists are immoral, unhappy or stupid.
We do not have to do everything that Stalin or Mao did. We do not have to exactly mirror even their successes, and we're under no obligation whatsoever to repeat their mistakes. "Stalin and Mao did some bad shit, Stalin and Mao were a communists, therefore communism entails doing bad shit," is no less fallacious than if we substitute "atheist" for "communist" and focus on their repression of religion.
It's ridiculous to assert that communism failed in the USSR and PRC due only to socialism's intrinsic weaknesses or its reliance on authoritarianism. The United States alone spent trillions of dollars (back when a trillion dollars was a lot of money) threatening the USSR and PRC with global thermonuclear war. Not because the USSR and PRC were repressive or authoritarian — the US loves them their authoritarian regimes — but because they were socialist.
It's hypocritical to mock communists on the one hand for being naive idealists and criticize them on the other hand for being willing to tolerate the messiness, problems and mistakes necessary to implement any kind of change — much less radical change — in the real world. It's just as hypocritical to condemn communists for wanting to rely on state power to implement communism and tolerate the use of state power to implement and maintain capitalist imperialism. If you're against state power in principle, destroy all government power and arm the populace... and let me know how that works out for you. It's simply moronic to condemn communists for having an "ideology" while pretending that your own ideology — Austrian or Chicago capitalism, liberal democracy, democratic socialism, anarcho-syndicalism, secular or religious humanism or whatever — is just a collection of good ideas, an "ideology of non-ideology" which has to be the stupidest slogan I've heard since I stopped debating cretinists.
If you're going to criticize communism and socialism, do so on the merits. There's plenty to criticize there: every sophisticated intellectual endeavor has its share of half-baked ideas and institutional conservatism, and only sharp and direct criticism can root out error. But please, if you're going to criticize communism and socialism, please try to use arguments and evidence more sophisticated than those used by cretinists and IDiots.