Hear, HEAR.
I think it's often a simple domestic moment like that, which is completelly absent of love and concern when even WE, the children of Ns, know finally, at last, without confusion, that love and concern is what we deserve....that is the last straw.
I had a similar moment, although I am not convinced my daughter's father was N, more like, just selfish and kind of nasty-tempered. He came home irritated at ME (not her) one night, and she was bouncing on my lap on a towel after her bath, happy and cheerful. He walked in, got one look at this cozy affectionate scene, and she stretched her arms up and went, Daddy, Daddy!
He looked at her, then me, and said to ME, I've had enough stress for one day, and walked outside to his "shed" and refused to come indoors and say goodnight to her, though she cried in her bed for 45 minutes: "What did I do? Why is he mad at me? Why won't Daddy come and say goodnight to me?"
The next day I told him we were getting divorced. He was willing to hurt her, to hurt me, and that penetrated my fog of rationalizations for staying.
Hops