Why did Jane Eyre attack John Reed?

Jane Eyre, the poor relation disliked by her Aunt Reed, was exposed to daily abuse from John Reed for years. Her aunt, his doting mother, turned a blind eye to it and would do nothing to protect Jane from her spoiled, sadistic son. The servants saw what was going on but were afraid to interfere. Jane, four years younger and smaller than John, had no good way to defend herself from this older boy, so she tolerated the abuse. As Jane explains:

He bullied and punished me; not two or three times in the week, nor once or twice in the day, but continually: every nerve I had feared him, and every morsel of flesh in my bones shrank when he came near.

In this instance, he finds her behind a curtain on a window seat reading a book about birds that he considers his own. He belittles her for being a poor relation and then throws the heavy book at her as a punishment for touching his property. When it hits her, she falls and cuts her head against the door. As she stands up, in pain and over her first wave of terror, all the anger that has built up inside her explodes. She thinks of John as a cruel tyrant and a slave driver and, for the first time, fights back against him in a frenzied way until she has to be pulled off.
In short, the anger she had stuffed down for years erupts. She can bear his abuse no more.