I'll counter and say that it's culture/conditions-based. Humans have a range of available/possible behaviors/thought patterns and they are reinforced/shaped by their surroundings/the system they live in. There are and have been egalitarian societies that aren't full of "mean, stupid, and crazy" people.

"The idea that the key features of successive societies and human history have been a result of an ‘unchanging’ human nature [..] is a prejudice that pervades academic writing, mainstream journalism and popular culture alike. Human beings, we are told, have always been greedy, competitive and aggressive, and that explains horrors like war, exploitation, slavery and the oppression of women. This ‘caveman’ image is meant to explain the bloodletting on the Western Front in one world war and the Holocaust in the other. I argue very differently. ‘Human nature’ as we know it today is a product of our history, not its cause. Our history has involved the moulding of different human natures, each displacing the one that went before through great economic, political and ideological battles."

"The world as we enter the 21st century is one of greed, of gross inequalities between rich and poor, of racist and national chauvinist prejudice, of barbarous practices and horrific wars. It is very easy to believe that this is what things have always been like and that, therefore, they can be no different. [...] The anthropologist Richard Lee [said]: "Before the rise of the state and the entrenchment of social inequality, people lived for millennia in small-scale kin-based social groups, in which the core institutions of economic life included collective or common ownership of land and resources, generalised reciprocity in the distribution of food, and relatively egalitarian political relations." In other words, people shared with and helped each other, with no rulers and no ruled, no rich and no poor. [...] Our species [..] is over 100,000 years old. For 95 percent of this time it has not been characterised at all by many of the forms of behaviour ascribed to ‘human nature’ today. There is nothing built into our biology that makes present day societies the way they are. Our predicament as we face a new millennium cannot be blamed on it."

-Chris Harman - A People's History Of The World: From The Stone Age To The New Millennium*

edit: and adding a short video https://youtu.be/Est6nay4Z5E?t=18

edit: some books that are on my TBR that might be worth checking out:

• Just Babies: The Origins of Good and Evil
• Hope for Cynics: The Surprising Science of Human Goodness
• The Social Instinct: How Cooperation Shaped the World
• Out of the Wreckage: A New Politics for an Age of Crisis ("shows how new findings in psychology, neuroscience and evolutionary biology cast human nature in a radically different as the supreme altruists and cooperators.")