D.O.A.

Marla Rakubian threatens Bigelow when he comes to her for information
The film begins with what a BBC reviewer called "perhaps one of cinema's most innovative opening sequences."[3] The scene is a long, behind-the-back tracking sequence featuring Frank Bigelow (Edmond O'Brien) walking through the hallway of a police station to report his own murder. Oddly, the police almost seem to have been expecting him and already know who he is.
A flashback begins with Bigelow in his hometown of Banning, California, where he is an accountant and notary public. He decides to take a one-week vacation in San Francisco, but this does not sit well with Paula Gibson (Pamela Britton), his confidential secretary and girlfriend, as he does not want her to accompany him.
Bigelow accompanies a group from a sales convention on a night on the town. At a "jive" nightclub called "The Fisherman," unnoticed by Bigelow, a stranger swaps his drink for another. The nightclub scene includes one of the earliest depictions of the Beat subculture. The next morning, Bigelow feels ill. He visits a doctor's office, where tests reveal he swallowed a "luminous toxin" for which there is no antidote. A second opinion confirms the grim diagnosis, and the other doctor implies that the poisoning must have been deliberate. Bigelow remembers his drink tasted strange.
With a few days to live at most, Bigelow sets out to untangle the events behind his impending death, interrupted occasionally by phone calls from Paula. She provides the first clue: a man named Eugene Phillips, who had been urgently trying to contact Bigelow for the last few days, had suddenly died. Bigelow travels to Phillips' import-export company in Los Angeles, first meeting Miss Foster (Beverly Garland) (whose on-screen credit reads "Beverly Campbell"), the secretary, and then Mr. Halliday (William Ching), the company's comptroller, who tells him Eugene Phillips committed suicide by jumping from his office a day earlier. From there, the trail leads to Phillips' widow (Lynn Baggett) and brother Stanley (Henry Hart).
The key to the mystery is a bill of sale for what turns out to be stolen iridium. Bigelow had notarized the document for Eugene Phillips six months earlier on behalf of Phillips' business associate. He connects Phillips' mistress, Marla Rakubian (Laurette Luez), to gangsters led by Majak (Luther Adler). They capture Bigelow. Since Bigelow has learned too much, Majak orders his psychotic henchman Chester (Neville Brand) to kill him. However, Bigelow manages to escape.
Bigelow thinks Stanley and Miss Foster are his killers, but when he confronts them he finds Stanley has been poisoned tooâafter having dinner with Mrs.Â Phillips. He directs them to call an ambulance and tells them what poison has been ingested so that, in Stanley's case at least, prompt treatment may save his life. Stanley tells Bigelow he found evidence that Halliday and Mrs.Â Phillips were having an affair. Bigelow realizes that the theft was merely a diversion. Eugene discovered the affair and Halliday killed him.
Halliday and Mrs. Phillips used the investigation of the iridium as a cover for their crime, making it seem that Eugene Phillips had killed himself out of shame. However, when they discovered that there was evidence of his innocence in the notarized bill of sale, Halliday murdered anyone who had knowledge of the bill of sale. Bigelow tracks Halliday down and shoots him to death in an exchange of gunfire.
The flashback comes to an end. Bigelow finishes telling his story at the police station and dies, his last word being "Paula." The police detective taking down the report instructs that his file be marked "D.O.A."

Question: Who shoots Halliday to death?
Answer: Bigelow.