She was very scared and angry when the other members of Mike's gang started picking on her. They are from a different neighborhood, a different gang, and have nothing in common. I'm sure she never understood why Mike took up for her and then took her home. Retaliation was just the way things were done. If you did something to one gang, that gang had to retaliate to make sure they weren't viewed as being weak. Mike was more than likely the only gang member she got a good look at so her friends would know who to go after.
This incident is important because Mike tells Bryon that he doesn't blame the girl for telling her friends. Mike says he had to do it because the violence has to stop. This makes a deep impression on Bryon throughout the book.
She tells them to beat him up because she is tired of all the white boys messing with her