"...Marx...was going to help the worker receive the just fruits of his labor. How could that be wrong?"

That begs the question: "What are the 'just fruits' of a man's labor?" Who decides? You should decide what you will sell your labor for. You. Who should decide what I will pay for your labor? Me. Just Fruits are what a willing seller and willing buyer agree for the exchange. No more. No less. If you don't like what I offer for your labor, look for another buyer. If I don't like what you charge for your labor, I can look for a different worker.

Interfere with that and the allocation of available assets is made more "sub-optimal." Note: The allocation of resources is always sub-optimal. The object is to make it more optimal, not less. The way to make the allocation of resources more optimal vs less optimal is to subject the allocation to more free choices...i.e., by reducing any constraints to producers, sellers, buyers and the exchanges between parties in any part of the process.

A free market is a free market whether you are a willing buyer in a buyers market, willing seller in a sellers market, not-so-willing buyer in a sellers market, not-so-willing seller in a buyers market, or either party in a "perfect" market where supply and demand match perfectly. That is how free markets optimize the allocation of resources whether the supply is below, equals, or exceeds the demand.

Marxism was founded on and is still based on the notion that with enough information, experts can run the political, economic and social systems better than the people can (i.e., left to make their own free choices). This notion was based on the Classical Physics ("Clockwork Universe" ) belief that "given enough information about all of the particles in the universe, we can predict the future state of the universe to any arbitrary degrees of accuracy." That science was (and is) wrong. Marxism only works on (and is dumber than) "a box-o-rocks."

The universe is, and the systems that Marxism seeks to micro-manage are, "fractal" or "complex" in ways that do not succumb to determinism. The systems that "work" are "emergent" systems. Godel's Law laughs at authoritarians...even though living under authoritarianism is no laughing matter.

Much better to look in the direction of Jared Diamond's "Optimal Fragmentation Principle" {“Guns, Germs & Steel” afterword, p458} "Innovation proceeds most rapidly in a society with some optimal intermediate degree of fragmentation: a too-unified society is at a disadvantage, and so is a too-fragmented society." Innovation drives real progress (vs "progressivism" ) and real progress grows economies, which make possible more freedom and a higher standard of living for everyone.

Trying to manage an economy (Marxism, cronyism, oligopoly, regulation, tarrifs, bribes, kick-backs, taxes, and everything else that interferes with free trade between free individuals and free states) drives out information about value judgments that buyers and sellers make in free markets. Those are precisely the value judgments required to "optimize" resource allocation. Given a choice, you won't pay more for something than it is worth to YOU, and the vendor won't sell his or her products or services for less value than they attach to them.

For an excellent (and thorough) treatment of that value proposition, I suggest George Gilder's "Knowledge and Power" (The Information Theory of Capitalism and How it is Revolutionizing our World).

Economics in Marxism is based on the Labor Theory of Value. Marx's economics was thoroughly decimated by Eugene Bohm Von Bawerk, a proponent of Austrian Economics. Austrian economics bases value, not on labor, but on its subjective Theory of Marginal Utility, a good but rather erudite account of which is in the book by Murray Rothbard entitled "Man, Economy and State".

Marx's economic theory does not contain a pricing mechanism. There is no means to determine prices. If you are just distributing production as equally as possible there is no necessity to know prices. I wouldn't even call Marx's theories economics. The definition of Economics is essentially the study of the most efficient use and distribution of scarce resources and without prices there is no means to determine what is needed by people and in demand. Someone just determines what he thinks everyone needs and then produces that. What if people have very little demand for a product? It doesn't matter someone has decided what is needed and wanted. Not the most efficient means to determine the use of resources.

The very fact that it requires the "right" person or persons in order to work means it is always doomed to fail, because no such person or persons ever existed or will exist.

Simple. The idea of having rulers control every aspect of peoples lives is in and of it self corrupt and moral degeneracy. Of course then we might have this discussion: Are immoral, degenerate, ignorant people capable of self rule? It's either self rule or be ruled. One or the other and every point in between.