The central twist: Laura isn't dead. Midway through, while McPherson is asleep in her apartment beneath her portrait, Laura walks in alive. She had been away in the country. The woman killed in her doorway was someone else, a model Shelby had brought to the apartment, whose face was destroyed by the shotgun blast. Laura's return makes her a suspect in the murder, and the mystery turns into what critics call a hallucinatory narrative.
Structural meaning: Once Laura returns, Waldo's voiceover disappears for good. She "returns from the dead to speak for herself." Looking back, this shows that the first half was a woman constructed by men's memories and fantasies, and the second half is her taking back her identity.
The killer: Waldo Lydecker committed the murder. He meant to kill Laura out of jealousy, rather than let her belong to another man, and killed the wrong woman. He hid the weapon in the clock. His line \"Murder is my favorite crime\" and his claim to be the only one who \"really knew her\" read very differently once you know this. In the climax he comes back to finish what he started, and his possessive love ends in his own death.
Hidden meaning: Waldo's obsession and McPherson's are mirror images. Both loved the portrait before the woman. The ending contrasts love that insists on owning her with love that has to accept Laura as a living, independent person.