First off...three cheers for Casey Affleck (and brother Ben's direction) in Gone Baby Gone.
Second off, Casey's character, Patrick Kenzie, crafted by author Dennis Lahane is an interesting main character for cinema.
He's interesting not because he's bold, brave, and under the radar but influential. No, what makes him interesting to me is he lives in a world of black and white, right and wrong, and certainly not in the world of relative morality.
(note, minor spoils may follow)
When Casey kills a child abuser, Casey's character is tormented by the fact that he has committed murder, and broken one of the ten commandments. He doesn't justify it by saying that the wrongs of the killed is worse that the wrongs of the killer. For Affleck there is no justification.
Ed Harris character, Detective Remy Bressant, is the perfect foil to show a moralistic relativity. Remy Bressant has frequently done off-the-records, off-the-books actions, and lived according to his own scales of good, evil, forgivable, and unforgivable, while Casey's McKenzie finds this thought process unsatisfactory.
In the film, this conflict between absolute truth and relativity plays out in a number of tough ways and in many ways everyone wants to bend the rules, except for this central character.
Ben Affleck's scripting is not as a morality play, but rather as a film that opens up these questions...even up until the final shots of the film.
The film leads people to ask, can you live in a world of right and wrong? And it not only leads you to ask what are the consequences in living this way, but it also forces you to ask, "what are the consequences of living in a world of relativity?"