I can only guess she thinks what MC labels as 'abuse' that she calls 'love' because of parents. As in, from one way or another, committing violence/abuse on your loved one = love. The parents could of done it to each other, one or both of them could have done it to her, it's the only thing I can think of that her twisted definition of love would make sense.
Even if we understand why, though, that doesn't excuse her treatment towards MC, to not have some awareness that how she had treated her boyfriend is different from the norm. She should have sensed something was amiss from the love stories she heard from her friends, if she even had any, but if she doesn't, then it solidifies how fucked up her situation is.
Even still, I have no sympathies for her.