Greed is selfishness. Selfishness comes from desire. And desire is very human indeed. Now in nature, would a lion think "If I don't share my prey, those other lions will die"? No. Greed is a survival tool, and in fact quite natural. The lion wants all it can get, because if it doesn't constantly strive for that, it could die. And lions certainly aren't the only ones. In nature, a person doesn't need 5 billion other humans to help it survive. So it really wouldn't care. It only needs a small tribe. Less than a thousand would do, or even smaller. At least to the mind of man in nature.
For Russia, communism was like a sugar rush. A wild, crazy, brief buzz, then a painful headache once it wore off. People got tired of it. People that were born to it didn't know anything else. They didn't know things could get worse; to them, things could get better. Some would want to make it better for themselves. This is called ambition, and it is the bane of any country trying to adopt true communism, and is inevitable - it's only human, after all. Communism is a great idea in theory. But in practice, people are just too human for it to work. Dictators rise. Capitalist endeavors work their roots back in, one way or another (see China). Things just fall apart.
The Soviet Union is gone, China is only communist by name, and Vietnam is going the same way.