The person you hit is the person who has just told you that you are not as wonderful as you thought.

 Via a Roy Henderson tweet referencing Evil by Roy Baumeister.  Page 141.

Is there is any connection between how people regard themselves and how they treat others? We have seen that groups that have a sense of superiority tend to be more violent than others. Does this mean that high self-esteem causes aggression?

No. At least, not by itself. Plenty of people have high self-esteem and don't go about harming strangers or taking advantage of their neighbors. High self-esteem, by itself, is not always or inevitably a cause of violence. As we will see shortly, most violent people have high opinions of themselves, but most people with high opinions of themselves are not violent. Violent people are an important but distinct, atypical minority of people with high self-esteem.

The most potent recipe for violence is a favorable view of oneself that is disputed or undermined by someone else in short, threatened egotism. The roots of violence lie in the gap between a highly favorable self appraisal and a bad appraisal by somebody else. The person you hit is the person who has just told you that you are not as wonderful as you thought.

There are two possible ways to react to a bad evaluation. 22 One is to accept it as correct, which means revising your opinion of yourself downward. In general, people hate to do this. Damage to self-esteem is usually accompanied by unpleasant emotional states: sadness, depression, disappointment, anxiety, shame. These are all feelings that are focused on the self, and they are miserable.

The other option is to reject the bad evaluation as wrong. This choice allows you to preserve your favorable opinion of yourself, and it is the one that people prefer. It does leave the evaluation hanging out there, however: How could that person say that to you, if it is not true? The usual answer is that there is something wrong with that person. He (or she) must be obnoxious, unfair, biased, stupid, or unreasonably antagonistic toward you. The most common and appropriate emotional reaction to such an evaluation is anger. Anger is unpleasant, but it is directed outward at the other person, not at yourself. That is how aggression gets started.

Is it possible that all the polarization and street riots and bitter protests have nothing to do with ostensible reality?  That they have nothing to do with their nominal arguments?

I am wondering whether all this acting out of anger and violence isn't, perhaps, a product of the education system which as spent thirty years relaxing standards and pretending that there are not hierarchies of performance.  That everyone should have exceptional self-esteem because everyone is a star.

And then utopian notions collide with reality when there is a mismatch between one's own expectations based on self-esteem stroking and one's own actual capabilities and the dreadful reality that people will in fact judge you by your performance, not your own estimated value.  

Whether it is ANTIFA/BLM riots, critical theory drones or social justice mewlers, they all come across as low capability.  If they are indeed enamored with their self-esteem and are angered by the refusal of people living in the real world to bend the knee to their own self-worth, then certainly there is likely some threatened egotism.  

Sure would seem to explain a lot of the manifested anger which otherwise seems to have no obvious source.  They aren't angry because of the nineteen unarmed African-Americans who are shot each year by police officers defending themselves.  The protesters don't care.  What they do care about is their own personal irrelevance and inconsequence to everyone else with a productive life.