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 find that when people talk about human nature, it's a really good way to find out what they think of themselves. (Hobbesian types make me really sad, except for the ones that are so up themselves that they think the category of 'human' doesn't apply to them.)