In the book Freak the Mighty, Maxwell Kane struggled through a difficult childhood. When he was four, his father killed his mother and was sentenced to life in prison. Max does not recall the murder, but later in the book, when his father tries to choke Loretta, Max remembers his father choking his mother.
Max was raised by his grandparents, Gram and Grim, his mother's parents. As a smaller child, Max acted out a lot of his issues impulsively by kicking. He even kicked in daycare. His actions earned him his nickname. He rejected affection and sympathy and kicked when people tried to get close to him. He also seemed to enjoy kicking versus other activities.
"I had a thing about booting anyone who dared to touch me. Because they were always trying to throw a hug on me, like it was medicine."
Max's behavior was most likely related to post-traumatic stress after observing his mother's murder and suppressing his emotions. As Max grew older he became a gentle person who withdrew from people.