I'm not trying to justify unlimited greed, nor do I mean that we should permit unlimited greed. I'm saying the opposite of that - I think we should have laws to curtail unlimited greed.
But I do still think greed and selfishness are human nature, because when I look at humans, I consistently see selfishness and greed. Maybe this means I'm a pessimist but I think it's just realistic. If humans weren't inherently greedy and selfish then there would be no need for laws that punish theft.
As for whether greed and selfishness have been dominant in human societies... it looks like the rich and powerful in society, throughout history, never gave up their power and wealth unless they were made to do so.