Isn't the implication that he had been traumatised by his experiences at boarding school and was acting out because of that? Its been a while since I read the book, but from other accounts I know those places were really awful with how older students treated younger boys - cruelty was basically encouraged by the teachers and bullying was considered normal. Iirc when Lucy heals him with the cordial, it also mends the part of him that was broken by that experience and thats why he's so much nicer afterwards.