Why are people so mean?

[Undiscussed Groomed for Male Slavery, One Victim of Many]

People are mean cause they've experienced meanness around them. Plain
and simple. You have to learn it somehow.

The above seems most clear. The below is confused exposition.

There are a lot of different forms of that. An interesting one is
where neither party is _trying_ to be mean, but is completely unaware
of the situation of the other party, and does things that hurt them.
Wars have been fought over such things.

Another funny one is where people learn to act friendly in ways others
consider mean.

But the common one is where your life is all messed up cause people
are mean to you, and so you learn to act mean to others as a norm. Do
what works! Often people believe here that they are reducing the
meanness of others by making it unpleasant to engage in, but
demonstrating by their own actions that the result tends to be
defensive aggression rather than submission.

I put "being hired as a mercenary" and "being mind controlled to mess
your community up" both within the category of "your life is all
messed up so you learn to act mean to others", but I've heard some
would disagree and enjoy their job.

Conspiracy theory: people are mean because it prevents them from
revolting against the current norms. Aristocrats are tired of being
beheaded. Get everybody upset early, when they don't understand
they're being oppressed, and then the people who filter through learn
no to act aggressively once they do.

Further conspiracy theory: Nobody is really acting mean, it's just a
bunch of oppressive influences and AIs making them sound mean to keep
the hackers from communicating with each other.