Communism

Nature, red in tooth and claw. The competitive drive is not uniquely human but it is human nature.

This is mostly unfounded. Humans are cooperative, by nature. The dominant ideas of society are largely driven by Mode of Production. Capitalism is competitive, ergo Liberalism is the dominant ideology, but this is only the case within the myopic frame of the last few centuries. Mode of Production has shifted with technological advancement, which has also shifted "Human Nature" with it.

Capitalism, for all its faults, tries to harness that nature for good.

Capitalism wasn't designed in a lab, but emerged out of feudalism. It doesn't "try" to do anything. Functionally, it services the maximization of profit, with little care for the wellbeing of society at large.

Every other system rests on an assumption of benevolence, either from the few (monarchies, dictatorships, oligarchies) or the many (communism, anarchies).

Incorrect, and unfounded. Please explain exactly why Communism rests on the assumption of benevolence, in any way, referring to the structures Communists wish to implement.

History has shown that assumption to be a fatal mistake.

More vague assertions, this time with the undertone that it is somehow ideas that drive history, and not Material Conditions. This is an anti-scientific, anti-Materialist assertion.