But a reckless rage had come over Harry. He kicked his trunk open, pulled out his want, and pointed it at Uncle Vernon. "She deserved it," Harry said, breathing very fast. "She deserved what she got. You keep away from me." He fumbled behind him for the latch on the door. "I'm going," Harry said. "I've had enough."
This quote occurs at the end of Chapter Two, shortly after Aunt Marge has acted so despicably that Harry was compelled to unconsciously blow her up. His magical powers have surpassed his control, and with this competence and confidence, Harry charges out of the Dursley house, not to be stopped by the confused dismay on the faces of the household; in other words, Harry takes initiative. He has always been rescued from the Dursleys, either by Hagrid or Ron, and here he helps himself out. The fact that Harry threatens Uncle Vernon with his wand places Harry in a quite precarious situation, as he will have to return to live with the Dursleys once again. But Harry doesn't care. In this scene, we realize the extent to which Harry has become a part of the Magical world. He will create his own entrances into it, and he will defend it at all costs. Harry has matured by this third book, gaining confidence and agency, and in this exchange we see it at this degree for the first time.