Neuropath by R. Scott Bakker
Tom's life is not what it once was. His marriage to the beautiful Nora is on the rocks and he now sees his two young children only on her say-so. His best friend Neil has moved to California to teach neurology. He has one success - a book on human psychology. Tom wiles away the time trying to teach bored grad students. But that all changes when Neil comes back into his life. For it seems that Tom's best friend was working for the National Security Agency, cracking the minds of suspected terrorists. Now it is Neil himself who has cracked and gone AWOL - what's more, he has left behind evidence that he has been employing his unique skills on civilians - obsessed with the idea that he can control the human brain.
Thus begins a terrifying sequence of events as Neil starts to kidnap and mutilate people with a connection to Tom. He damages their brains and then releases them - often leaving them mad. But only when he gets near his ultimate target does he reveal the full horror of his plan . . .
The last page of the book has the scene with the child:
'Mommy?' a little girl's voice whispered.
Thomas heard the sharp intake of Nora's breath.
'I love you, Mommy.''I love you too,' she rasped.
'Yesss,' Ripley said.
'I really, really, love you…'
The words were right, but the world that gave them meaning was so very wrong. Soon, Thomas realized, his son would awake also.Then the screaming would begin.
'I luvvvvvv…' his daughter cooed in a smiling, teary-eyed voice.
'Shhhh,' Thomas croaked. 'Time to sleep, honey.'