If they re made to feel stupid, they react with force. Their perception of their own capabilities is severely warped. Any threat to their already fragile egos can lead to anger and violence. Simple arguments between two people can escalate to anger that is out of control. Harming someone physically can bring such relief to the perpetrator.
Physical abuse is often an inability to express the emotion of anger.
We all get angry, but most of us find positive ways to alleviate that anger. Anger management is a prerequisite for an emotionally healthy individual and a healthy relationship with another person. Individuals who are physically abusive are unable to self-regulate their anger response. Their reaction to something that someone does to them is similar to a runaway train. It is difficult for them to stop, once that anger is unleashed.