When Max finds out from the school principal that his father, Killer Kane, wants to see him when he gets out on parole, he starts panicking at school. He covers his ears so he can try not to hear the principal and he goes berserk crying and cowering in a corner until he passes out. He comes to and thinks he had hurt someone because the nurse is crying. She assures him she hasn't. Max has trouble coping with anything that has to do with his father.
Freak's father kidnaps him on Christmas Eve and ties him up. His girlfriend Loretta tries to rescue Max when his father catches her. He starts choking her and Max begins to face his memories. He yells at his father to stop chocking her and tells him that he remembers when he killed his mother. He tells him that he saw him kill his mother. He had never forgotten. In his mind he feels like he has become four again and is watching his father killing his mother.
The scenes mentioned above serve as a revelatory moment for the reader because it is something Max himself hid from, and didn't reveal to us until he was forced to by events. One purpose the memory of his mother's death is to hit the reader very strongly, to shock us into feeling deeply empathetic for the boy, and to bring about feelings of hatred and contempt for his father. It was a mystery we were dying to learn the truth of since the beginning. It also enhanced characterization and made a lot of things clear about who Max was. It also furthers the man vs. self plot as Max has to cope with memories of something he didn't want to face. It's been something that has been eating at him since he was very young, and he's finally released from feelings of guilt, self-repugnance, and shame for fearing that he was a "chip off the old block."
Correction about the above comment: Freak's father took off shortly after Kevin was born, despising him for his medicla condition. It was Max's father who kidnapped Max.