Jimmy Porter's anger stems from feeling alienated and helpless in a rigid class system. The real power and opportunities are for those people who were born into money and have the right family connections. These are the people who get treated better than everyone else just because their families are the "in" crowd of old money and influence. Jimmy is from the working class and isn't given the same opportunities as those from the "Establishment". The opportunities for power are not available to him because he isn't from the right family. He sees people like Nigel, who is "stupid and insensitive", has no beliefs of his own, but will make it to the top just because he was born into a wealthy family with connections. This makes him angry. He has a degree, but it isn't from a respected university. The social and power structure of society is closed to him, and Jimmy can't accept this. As a result, his anger leads to violence. He doesn't seem to fit in anywhere, and he lashes out in anger because he feels helpless to change the way it is.
The relationship between Violence and Power in John Osborne's play Look Back in Anger is an interesting one. It seems more like an inverse relationship. Jimmy Porter is angry and violent because he feels helpless. If he had power he wouldn't need to be violent. When he was ten years old he watched his idealist father dying for a year from wounds received fighting for democracy in the Spanish Civil War, his father talking for hours, "pouring out all that was left of his life to one bewildered little boy." He says, "You see, I learnt at an early age what it was to be angry—angry and helpless. And I can never forget it.''
He feels so alientated from any power that his only response is anger/violence.