Hurt people hurt people. Abused people abuse people. It is a self-perpetuating cycle, unfortunately. I have never ceased to be amazed at how mean, let alone abusive, parents can be to their own children. Once a child has been taught to be mean by their own parents, I don't think they ever fully recover.
Biological benefits? Well, if you consider emotions to be biological, which I do, I would say the benefit is emotional survival. When one is mean, one shuts oneself off from feeling, other than the anger that is directed outwards. When one ceases to feel, then one ceases to feel one's own pain, as well.

Because they're angry, unhappy, and/or in pain, and have not learned healthy coping skills.

Eat or be eaten, is how some people see it. They may grow up some day.

The Selfish Gene.

My theory is simple. People have lost a connection to something positive and meaningful in their lives. We need myth, magic, stories, something outside ourselves to believe in. For many Americans that has become people like Trump and others who embody selfishness and the idea that value is connected to material wealth and your superiority to others. While people like him are not new the Internet allows for a frightening distillation of such attitudes. Living in a echo chamber of far right nationalistic rhetoric or the same but far left is not healthy. Other humans become nothing more then bad memes or sensationalist stories of rape, violence and depravity. They become the enemy even though one has never met them and knows nothing true about them. I escape to an extant simply because I and I think many on this site are an anomaly. We are the outsiders and while we may have the advantage of perspective, knowledge and in many cases education we are far outnumbered and out gunned (in many ways) by the masses. This is why sometimes I become annoyed at our position of superiority as it means nothing besides a personal fantasy to solve the real world problems presented to humanity. Humanity needs a shared mythology based solely on being here on this planet among this species. Nothing else should matter; agnostic, atheist, religious etc. Actions and reason should rule policy and yet something more if the weight of history is any indication needs to replace religion/dogma/nationalisim/racial identity etc. Every group of people within their minority finds ways to oppress their own people. LGBTQ has homosexuals attacking trans or butch lesbians attacking femme lesbians. African Americans attack other African Americans for dating or marrying outside their race. Of course white folk as the majority at least for now exhibit bias and hostility towards minorities not only outside "whiteness" (whatever that may mean to them) but also attacking other whites for all sorts of reasons. Ones neighborhood, ones dress, economic level, attitude towards things like guns, economic status. The point being all of these bias are at the end of the day a bullshit excuse to make oneself feel better at the cost of others. Rather then through self reflection and self improvement. So as for your question why are people mean? It is easier then being nice. Now a side note I think the idea that such attitudes are systemic are incorrect. The media makes every instance of hate, violence, bigotry feel like it happens every day everywhere. I think most people just want to live and be left alone. But that is not good news coverage and does not drive ratings or advertising revenue. Are there biological benefits? If you teach or are taught to enjoy being mean and build those physical pathways in the form of neurons in your brain then the benefit is the release of endorphins and a form of happiness. This is why it is so important to raise children with a sense of empathy and kindness not just towards other humans but all living things.

Thank you all for your replies. I take pleasure in seeing others happy. It hurts me if they hurt. Not even people...animals too. Yesterday I had to stand a student down off the BSc nursing course. As a Nigerian, she will also lose the right to remain in the UK. It was heartbreaking to see her so distraught. She had lied on her initial application and her English was very poor. Still its not nice. My colleague thought it was funny. Then my partner's son decided to let us down at the last minute. My partner hasn't seen him in years and it was a surprise. I wouldn't do that in a million years. I just do not get unkindness. I do not understand people who take pleasure seeing someone hurt, even someone as onerous as President Trump.

It can be a bit of a knee jerk reaction in response to how a person has been treated by others previously, I think it goes back to a primal time when it was a fight for resources that determined the survival of you and yours, but usually these days at least after the initial interaction, empathy takes hold, but many times it's a delayed reaction and as most of our public interactions are momentary we don't get to see the human side of anyone, it's a snap judgement and we move on before we can even think to care. Maybe one day we will develop a less reactionary response to others but it seems we just aren't that far yet.

Mean people are miserable... Assholes are miserable...

Not everyone is unkind to others. No tangible biological benefits from being unkind exist.

Protection of the tribe. A necessary intent for the tribe as it has been. May still need to maintain status quo. But, may need to learn new.

This balance is difficult to determine. The immigration crisis in Europe of the past 6 years requires depthful analysis of what to do and what not.

The US has NOTHING in place to help immigrants to become one of us. This is wrong for everybody.

Fear, jealousy, sense of frustration, the reasons are myriad. It's a petty victory from a weak sense of self importance.

We are an incredibly flawed species, and that is being generous.

Because people suck! So suckage can be passed on!

Self advancement.

Exactly what I have found....

I am reading the book The Selfish Gene by Richard Dawkins. In it he argues that all organisms are simply vehicles for the genes they contain. The objective of the genes/set of genes that coexist (in one body), is to survive and multiply (by having children). This process of survival is often faced with interactions with other genes/sets of genes. This could with members of the same species or different. Inter species conflict is understandable, but professor Dawkins' line of reasoning sheds light into why selfishness may be genetic. Of course, there are other forces at play, which is why a certain amount of selfishness and certain amount of altruism, help maximize the survival of said genes.

They’ve given up

Part of it can be that thing of tribal instinct overriding rational thought.

Nothing that im aware of.

If all treated equal then no unkindness,