Honestly just a blind guess perhaps it’s because his crush on Catherine i believe first day of y school (the one he currently in) and she didn't like that so started bullying him because he said they marry or he agreed to marry her can't remember which then gets a crush on some else and some extra evidence is that Julia said can't remember the exact words but basically that she developed early beyond her years that kinda thing so she likely took the marriage idea seriously and felt betrayed. On I think day 2 where they get put into groups she clearly hates Catherine likely partly blames her but mainly blames Mc for betraying her. But I'm not 100% sure likely got some things wrong or it's more complex than that but we will find out in a month or two so not that long and we should get an answer. Feel free to correct or add anything as I'd like to know
That's basically my take on things, that I had for quite a while.
Mc and Vic as childhood friends spend a lot of time together, competing and playfighting often. Vic starts to adore mc more and more as he keeps beating her in their competitions and her respect for him grows larger larger. She mentally grows up faster than him and makes him promise to marry her in the future. She is more grown up and is IN LOVE with him so she takes it seriously. He is still mentally a child that can't really tell the difference between friendship and love so he doesn't take it seriously.
Much later they start their new school together, mc meets cathrine and falls for her. He sees Vic as his best friend so he tells her and she gets furious, because she sees it as a betrayal of their promise.
As mc grows up, he becomes more and more of a pervert, openly staring at all the girls. Vic sees that and it only serves to make her even angrier at him. At some point she starts to bully him, which starts a vicious cycle.
Vic's bullying makes mc feel weak -> he becomes more meek, becomes more of a loner and flees more and more into videogames and porn. -> Vic sees Mc's weaker self as a mockery of the person she fell in love with and who she looked up to -> Vic becomes even more angry -> Vic bullies mc even harder -> repeat.