The reality of communism is at least 50 million and perhaps as many as 100 million dead in the twentieth century...
Well, that's communism as implemented by assorted demagogues, not as Marx and Engels wrote about it.
"From each according to his ability and to each according to his need" is an absolutely wonderful idea.......if you totally disregard human nature.
That's the key point, I think. Marx and Engels were, among other things, hopelessly idealistic, and the fundamental idea of communism, which is as you expressed it, is deeply contrary to human nature, and that's why it failed. Nobody ever implemented communism as Marx and Engels imagined it, but even if someone had, it wouldn't have worked. Marx thought it would happen first in some place like the United States or Britain, a developed industrial economy with a large working class, but he failed to foresee the rise of labour unions and how they would short circuit and preempt his ideas. Instead it happened in rural agrarian economies and the dictatorship of the proletariat emerged as a dictatorship of just another elite group, consistent with human nature and, in retrospect, completely predictable.
Marx was wrong. It really is that simple.