10. Hide and Seek
It turns out that his identity disorder had been triggered by finding his wife cheating on him.
It’s certainly plausible that a person with multiple personalities could be violent in one personality and calm in the other.
It is even plausible that if the violent personality did commit murder then it could keep that information from the other personality. What is less plausible, however, is that the violent personality could hide the fact that it committed murder from the police.
The problem with the concept is that even though the personalities are different and separate, they still share the same DNA, hair follicles, and other biological features. The physical evidence would point toward the single person, no matter which personality was in charge at the time of the crime.
What is more, schizophrenics can’t control when one personality manifests itself. It would be impossible for the violent personality to actively hide from the police by manifesting only as the other personalities. The break would be obvious.
Despite the technical problem regarding the psychology of split personalities, the movie is still very good. In fact, then ending really is logical, so long as you accept that the police are buffoons, so you can forgive the writers if they took a few liberties with the plot. Besides, Robert DeNiro is the lead in the movie, so you can’t really go wrong.