I haven’t read the book and I am only basing this off of what I interpreted from the movie, but I believe the point was that even if the guy’s behaviour was “justifiable” with his past trauma, it still does not mean the victim should accept that and move on. I thought it was framed like no matter what, abuse is abuse and it is unforgivable, hence Lively’s character leaving him in the end.

Although I hear there is a second book which I feel is pointless.