In the beginning he wanted to beat him once but I guess during the hs years they form an unlikely bond of friendship. Although Lee Sangmin hurts him, in his own twisted way, he thinks of Kim Jihyun as his friend. But for him, being his friend is secondary and getting success and taking revenge on father is primary. I think if Lee Sangmin was put into therapy from the beginning, a lot of this could be avoided. The father is truly the trashiest character in the novel
By the way, does anyone know if Kim Jihyun ever realizes that Lee Sangmin has schizophrenia?