Generally this is sold as a romance, those in the know will tell you it is not, it is a story about a woman falling in love, and said love becoming abusive.

It is not any of these things. 

It Ends With Us is Colleen Hoover's way of saying that everything her mother went through, everything all abused women go through simply doesn't matter because "he was a really good dad though :((("

It's clear for the snarky remakes Lily gives about her mother staying, the abuse she suffered, how she justifies what happens to her, how the fucking abusive and

almost literal fucking rapist

man get's to be a father to his little girl. Any woman who would still consider a man who beat her mother a person isn't a person themself.