One of the main problems of Noir is that there’s no room for a happy ending. It generally differs from crime fiction in that the protagonist isn’t trying to solve a murder or get away from a murderer – he IS the murderer, trying to get away with it!
This is more satisfying for the audience, bringing it closer to the act of violence. Since we are bound to identify with the protagonist, a happy ending would mean that he got away scot-free, something that’s simply not in the cards.
You may succeed in evading the Law, but not the Hays Office! This leaves only two possibilities: Either he really didn’t do it, or it was all a dream, both being of course pure nonsense.
NO MAN OF HER OWN is a prime example of this conundrum and its highly unsatisfactory solution. Barbara Stanwyck is pregnant with an unsympathetic father, his idea of child support being a train ticket out of town.
When the train is wrecked, she ends up in a hospital with another pregnant woman’s wedding-ring on her finger, and the husband dead. Since his family has never seen their daughter-in-law (or their grandson) Stanwyck assumes her identity as preferable to indigence – not a bad idea altogether, if stretching the suspension of disbelief a bit thin.
Enter the baby’s real father with a blackmailing scheme making it necessary for Stanwyck to murder him, not to jeopardize her new-found happiness with the dead man’s brother. So off she goes to death row.
No she doesn’t, because her adopted mother-in-law confesses to the murder, and besides the victim was already dead, which Stanwyck knew all along, she just wanted to put a hole in his mattress! Say what?