Some of disagree - that which is just can be natural (though not all that is natural is inherently just).
I believe that human nature is flexible; it's not set in stone. Anything can be part of human nature. All they need to do is adapt to it. Therefore, arguing that something is "human nature" is really disastrous, because it's used to justify terror and exploitation--"it's the only thing that works." It's probably the cheapest argument I've heard for the "justification" of capitalism. It was also used as an excuse for all sorts of tyrannical and oppressive regimes.
We need to stop using "human nature" as an excuse to fight for/justify systems, but rather, try to argue that it's the most just system. Everything can be human nature, so it's not an argument for any political or economic system.
You might call me a naturalist because I believe that justice is part of human nature, but as I said, I believe that every sort of injustice is part of human nature too.
That being said, I do believe that as human beings are all conscious beings, they have the right to be equal and are of infinite value. But that's not "human nature", because humans don't automatically act on it. The problem is that different people have different beliefs of natural law and inherent rights, so fighting for things on the premise that they're "natural law" is useless. Everyone says, "This is natural law," "No, this is natural law." It is an excuse for tyranny. If you truly believe in justice, you don't have to advocate it as natural law. "Natural law" is the last resort for injustice. Even though I believe anarchism is natural law, I don't convince people into it by arguing that it is. I convince people into it by arguing that it's the most just system. Tyrants can't argue that their system is just, so they just say that it's "natural law".
To summarize, I believe in natural law, but that's not enough to justify the system I advocate.
Economic Left/Right: -9.75
Social Libertarian/Authoritarian: -9.28