Cause the author needed a "strong" reason for the MC to forgive the douchebag, so she kill her so she could get back with him.
Makes so much sense but why couldn't she end up with the silver haired guy. Not that i like him so much but it's better than a ducking douchebag who abandoned her for YEARS. (╯°Д °)╯╧╧
IMO she doesn't need a man to be fine, just keep hating the damn bastard that betray you and refuse the other psycho, and live her life to her own drums. Then, along the way maybe find some decent partner if needed. But that's not how any of this works, does it?
This story started to drag after the whole changing bodies stuff. It was perfectly fine before that why would they destroy an already fine story just to keep it going.