Why do you work on the assumption that humans are inherently noble?
We are not.
Or instinctively work for a common good?
We do not.
We desperately want to transcend the confines of our animal ( sorry, animals) state, and some do. But not enough of us to think of humanity as a collective.
Bees and ants do better than us.

Animals kill only to eat. They cooperate to hunt. We kill with utter abandon.

Our battles for individual survival - water, food, resources begin with tribalism and ultimately as resources dwindle, it’s each him/her self.

Fantasy writers and Hollywood got it right.