'Human nature' is whatever people want it to be for the purpose of argument. It's a thought-terminating cliche. There isn't one single personality type, although there is some evidence to suggest that there are 4 or 5 basic ones.
Greed is part of human nature, sure, but so is empathy and selflessness and a sense of justice. We certainly didn't need to build a society that rewards people for giving into one of their most selfish and cruel impulses.
