Why does the author include the childhood flashback that is on pages 41–43 of Monster?

Monster, by Walter Dean Myers, is a young adult novel which tells the story of sixteen-year-old Steve Harmon. Steve is awaiting trial for the murder of a man during the armed robbery of a drugstore.
The flashback scenes you refer to show Steve four years earlier, when he is twelve years old. He and his friend Tony are walking through a park throwing stones at lampposts when one of Steve's stones accidentally hits a woman who is walking by. When confronted about what they have done and Tony is punched, Steve does nothing to back up his friend and does not accept the blame.
This flashback gives the readers a clue to Steve's character; he is okay letting somebody else take the blame for something that he has done.