Responses
From my understanding, backstories like these are not supposed to, nor should they, defend a character's actions. You can't really justify sexual violence with a sob story.
But it does explain though. Basically to someone like JJK, martial arts and competitions were his escape from his hellhole apt with his dad and also gave him the means to move beyond poverty. It illustrates why JJK is so hellbent on winning his matches and training even while injured. To him, its a sort of reprieve.
Girl.... I saw the backstory and its explaining why he likes older ppl and hates alcohol, but not why he is such an asshole to kim dan and other ppl (its not really defending him from his current actions, esp in season 1)