I could easily cite Chile, Guatemala, Nicaragua, EL Salvador, Argentina, Bolivia, Paraguay, and countless other 20th century totalitarian regimes as instances where "capitalist experiments were doomed by greed and sociopathy." Very easily, given that these regimes were backed by American capitalist business interests. But realistically? When you're strong enough and ruthless enough to take over an entire country, you're probably more interested in power than in ideology, one way or another.
If we're not there yet, what do we do when we are?
To me socialism is just an inevitability. We will eventually reach post-scarcity. I think if we can examine the current system and find so many ethical problems... It's worth asking ourselves what we could do to get to that ideal star trek society faster.
Russia and China were not developed countries prior to communism. They didn't have what we would consider the rule of unarbitrary law.
Marx and Engels saw socialism as the next step a society would move into after capitalism. They predicted this had to rest on the foundation of a society which had already experienced it's 'bourgeois revolution', meaning power had to leave the aristocracy and be in the hands of the capital owning class.
They saw democracy as a necessary step for a society to progress to socialism. There is no great examples of this in the 20th century. If you read even a little Marx, how could you ever expect the kind of society he describes without first having democracy? Socialism is based on one concept: democracy in the work place. If democracy does not exist in the first place, the endeavor will always be doomed.