Damn, I actually called it.
The R18 chapters make it clear he genuinely hates her (but does feel something for her) and she all but welcomes the abuse, as long as it's MC that's doing the hurting it's okay. She's just that broken of a person and feels she needs to suffer for MC to forgive her.
To those flipping out, the signs were there all along. You ignored them, didn't pay attention, or never read the R-18 chapters which this author treats as canon. (The two of them have actually been having constant sex all this time, with him being repeatedly rough with her)
This story told you what it was going to be about, do you expect everything to just have a happy romantic ending?