Brief synopsis of the scenario just in case you want to talk about this but haven't seen the movie.
Basically Doyle "kidnaps" Amanda in order to give her a better life, because his own daughter was killed. It's sorta his way of making up for all the other kids he failed to save during his tenure. Patrick figures this out, and at the end, he can either let Doyle keep Amanda, or call the police and return her to her mother (who's a drug addict, horrible parent, etc). His girlfriend wants him to keep Amanda with Doyle because she will have a better life.
He ends up deciding to call the police and give Amanda back to her mother. His reasoning is that he wouldn't be able to live with himself knowing that a girl was taken from her mother, even if she might have a better life, one day she might grow up and wish she knew her mother - and he would have been able to fix that situation but never did. It's basically a matter of principle to him, that he can absolutely never bend... even if it means that Amanda will end up being worse off living with her mother.
Do you agree or disagree with his decision?
Personally, although I would be TORN if actually faced with something like this, I probably would have done what he did. I think that if you don't live by your principles you have nothing. And I also think that at the end of the day you have to do the right thing. What if her mother changes, and ends up raising Amanda well? Obviously the odds are stacked against that... but you never know.