Dressed to Kill (1980)

Pooper: (Thanks Curt, The Grand Poop-bah)
Dr. Elliot (Michael Caine), dressed in his woman mode, is the killer.
Long Ending:
Kate Miller (Angie Dickinson) is remarried to a man who, while loving, leaves her sexually frustrated. They live with her teenage son Peter (Keith Gordon) from her first marriage (Peter’s dad died in the military). Kate attends therapy with Dr. Elliott (Michael Caine), where she discusses her lack of fulfillment and desire for an affair. She asks if Dr. Elliott would sleep with her. He admits that, while he finds her attractive, he wouldn’t sleep with her due to being married and ethical. Dr Elliott instead urges Kate to discuss her dissatisfaction with her husband. Instead, later that day, Kate meets a mysterious man at an art museum. The pair get into a cab together where the man performs oral sex on her. They go to his apartment where they sleep together. Unbeknownst to Kate, a stalker has followed her there.

While her conquest sleeps, Kate rummages through the man’s belongings and finds a doctor’s note saying he has a sexually transmitted disease. Overcome with both disgust and guilt, Kate hurries out of the man’s apartment and into an elevator. There, her stalker (a blonde haired woman) attacks Kate with a razor blade. The elevator stops during the attack and a woman named Liz Blake (Nancy Allen) witnesses Kate die in front of her when the doors open. Liz also sees the face of the assailant in an elevator mirror. A maid (who also saw the crime scene when the doors opened) calls the cop. Detective Marino (Dennis Franz) tells Liz that nobody else saw the blonde woman and that unless Liz can corroborate her story soon, Marino will arrest her for the crime. Marino is especially skeptical of Liz’s story as Liz was at the apartment working as a high-end call girl (prostitute). Meanwhile, Dr. Elliott receives a message from an old patient named Bobbi. In the message, Bobbi (who is transgender) claims to have followed Kate when she left her appointment with Dr. Elliott and killed her as revenge against Dr. Elliott, who has refused to authorize Bobbi’s desired sex reassignment surgery.

Liz is now being stalked by Bobbi, who hopes to kill the only witness to her crime. Bobbi chases Liz into the subway. Bobbi corners Liz, but is prevented from killing her when Peter intervenes (although Bobbi successfully escapes). Peter had been independently investigating his mother’s murder and was following Liz so he could propose that they combine their efforts. Together, they determine that Bobbi was a patient of Dr. Elliott’s. They tell Detective Marino about their discovery, but Marino responds that Dr. Elliott isn’t cooperating so he can’t corroborate their story. Marino essentially says that to prove their claim (and keep Liz out of jail), they’ll need to steal Dr. Elliott’s log book and prove that Bobbi exists and shared the same doctor as Kate. Meanwhile, Dr. Elliott visits Bobbi’s current psychologist – Dr Levy. He confides his suspicions that Bobbi may have committed murder, and Dr. Levy promises to interrogate Bobbi at her next appointment.

That night, Liz visits Dr. Elliott under the false pretense of needing therapy. Peter hides outside the office and observes their interaction. Liz begins seducing Dr. Elliott, telling him to get undressed while she goes into another room to freshen up. While in the other room, Liz finds the log book. Meanwhile, Dr. Elliott gets undressed… and then puts on a wig and female clothing. It turns out that Dr. Elliott is actually Bobbi. As Bobbi prepares to kill Liz, Peter (who witnessed the whole thing) tries to help. To everyone’s surprise, an undercover cop emerges and shoots Bobbi (knocking Bobbi out). It turns out that Marino assigned an undercover cop to follow the duo and intervene if help was needed.

At the police station, Peter and Liz meet with Dr. Levy. There, Levy explains that Dr. Elliott is transgender, but in deep denial of this fact. This resulted in Dr. Elliott’s feminine side (Bobbi) to develop into a separate identity (basically, Elliott has multiple personality disorder). Bobbi was indeed angry that Dr. Elliott wouldn’t go through with the surgery. And so whenever Dr. Elliott got sexually aroused (first by Kate, then by Liz), Bobbi would emerge to kill the women so that Dr. Elliott would remain sexless. When Dr. Elliott visited Dr. Levy and said Bobbi was the murderer, that claim essentially acted as a confession that he (Dr. Elliott/Bobbi) was the murderer. Dr. Levy went straight to Marino to turn Dr. Elliott in, but by that time, Liz and Peter had already enacted their plan (and thus, why Marino sent the undercover cop to find them). Dr. Elliott is confined to a mental hospital. And although Liz has escaped alive, she suffers from severe trauma from the experience (including having a vivid dream that Dr. Elliott/Bobbi had escaped and was still hunting her down)