Gone Baby Gone is not a movie about kidnapping or unhappy family, but about a difficult choice that the main character has to do. Patrick Kenzie faces a great moral dilemma. On the one hand, he finds out that a girl who is supposed to be kidnapped and dead lives happily at his friend's place. On the other hand, he strives for delivering justice and keeping his promise to return Amanda to her mother. He chooses to call the police, return the girl and maintain the justice, but at the same time, he returns Amanda to a woman who is rather concerned about her daily needle than her own daughter. As a detective and law obedient citizen, Patrick takes a right decision, but from the deontological point of view, his act is immoral and breaks all three maxims of categorical imperative.
Acting with a good will is fundamental for Kantian ethics. Undoubtedly, Patrick has a good will and is convinced that his action would be beneficial. However, first of all, he is willing to make benefit to himself not to Amanda. He decides her destiny without her participation. It breaks the first premise of deontological approach as Patrick's conduct is not right for Amanda in her circumstances. Amanda lives happily with Doyle who "kidnapped"her because her mother does not cares about her and does not bring her up properly. Basically, Patrick's decision to take Amanda back home ruins her life and hopes for the better future. In this case, his action is immoral not only from the Kantian ethics standpoint, but also from the utilitarian one.
Additionally, Patrick's morality fails in the second maxim of Kantian ethics theory. He is complacent that he has acted in a legal manner and revealed kidnappers, but he is not concerned about the consequences of his deed. Hence, deontology is not as helpful as utilitarianism; nevertheless, it illustrates that Patrick is not really interested in happiness and wellbeing of Amanda, he cares much more about his reputation and ensures the delivery of justice. At the same time, even his own principles of justice fail because Helen's conduct does not allow her to be a proper mother for Amanda. She neither cares about her upbringing, nor about her own drug addiction. Basically, Patrick uses Amanda as a tool. Amanda's happiness is not his target, and so-called rescue of the girl is just the means of maintenance his conduct.
The main trigger that urges Patrick to inform police about Amanda's location is a promise that he has given to Helen McCready and his citizen standpoint. Duty is an important concept of deontological ethics, but his duty for Patrick is rather torturous and involuntary. Putting Captain Doyle and breaking up with Angie out of his sense of duty to Amanda's mother.
Therefore his action cannot be moral with regards to Kantian ethics theory as soon as the main character does not apply his own principles of justice. Actually, his action depends on his egoism, soulless consciousness and blind adherence to his annoyingly proper principles that are not pertinent in that situation. These traits are not those that establish a universal law, and it breaks the last premise of deontological ethics.
Patrick's action is immoral with regards to Amanda and deprives her of a normal life. Patrick watches Amanda in the end of the movie, but it does not mean that he has ever cared about her future and tried to make good will decision. He is mainly concerned about his duty to justice and Helen, but not to himself. In this situation, his action is not moral neither to Amanda, nor to himself as an individual because he acts against his will.