People who use the notion of "human nature" to justify capitalism have a lazy, unfounded argument. Human nature is a malleable concept and to believe that greed is a quintessential example of human ambition and thought is unjustified and tragic.
@LaLuchadora I also find it a very unimaginative and pessimist view to take. Humans have constantly improved their lot in this planet through advances, both scientifically and socially, is it so impossible to imagine a world without greed given that we were all living in caves an astronomically insignificant amount of time ago?