Notorious - Ending Explained

The film's tension culminates in the revelation that Sebastian is poisoning Alicia. Devlin rescues her, carrying her down the grand staircase in front of Sebastian and his Nazi associates. Devlin bluffs, claiming she must go to the hospital, and Sebastian is forced to play along to hide his blunder from his coconspirators.
The Twist: The true horror of the ending is not a gunfight, but a door closing. Devlin locks Sebastian out of the car, leaving him alone on the steps. Sebastian turns to walk back into the house, knowing his mother is inside but also that his Nazi colleagues are waiting to interrogate (and likely kill) him for marrying an American spy. The "villain" is left to a fate worse than arrest, abandoned by the woman he loved.

The 'Pimp' Narrative: Some modern critics interpret the film not as a romance, but as a dark story about a man (Devlin) essentially pimping out a woman (Alicia) for the state, exploring themes of misogyny and exploitation within patriarchy.
Sebastian as the Hero: Another reading views Alexander Sebastian as the true tragic hero; he is the only character who loves unconditionally and honestly, while the 'heroes' lie and manipulate him.
The Erotic Thriller: Critics have noted that the plot about uranium is meaningless (a MacGuffin) and the film is truly about the sexual politics of the bedroom—impotence, jealousy, and the 'key' to sexual secrets.