Edit: I checked the R18 one Chapter 37. if it is canon and the others' theory that each chapter happens in between then he finally has dirt on her that he may use as contingency or if he decides to continue his revenge. Maybe it also made things feel 'equal' for him since the girl also had the same thing when she bullied him.
In retrospect, at the end of the last chapter we see MC is all beat up and bandaged again. I wonder if he ended up fighting wither bullies to defend her before saying he didnt forgive her.