"Death of a Demon" is a Nero Wolfe mystery novella by Rex Stout, first serialized in three issues of The Saturday Evening Post (June 10, 17 and 24, 1961). It first appeared in book form in the short-story collection Homicide Trinity, published by the Viking Press in 1962.

Lucy Hazen has a preemptive confession to make to Nero Wolfe – having come to despise her husband Barry, a cruel public relations counsellor, she has recently become plagued by thoughts of shooting him with his own gun. In order to deter herself from following through on this impulse, she has decided to confess this to Nero Wolfe, knowing that if she did commit the crime he would reveal the act to the police. Although bemused by the meeting, Wolfe humors her and agrees to show her his orchid collection, but while they are upstairs Archie Goodwin hears on the radio that Barry Hazen’s body has been discovered in an alley, shot in the back.

Despite Lucy’s confession, Archie is convinced by her reaction when he informs her of her husband’s murder that she is innocent of the crime. Wolfe and Archie learn from Lucy that she last saw her husband at a dinner party held the previous evening for a group of his clients – Mrs. Victor Oliver, Anne Talbot, Jules Khoury and Ambrose Perdis – and his copy-writer Theodore Weed, whom Lucy clearly harbors feelings for. Although similarly convinced of her innocence, Wolfe is reluctant to accept Lucy as his client and sends her away, though he keeps the gun in his possession for safe-keeping. Using an old mattress, Archie acquires a fired bullet from the gun and turns it over to Inspector Cramer for comparison.

Lucy is detained as a suspect in her husband’s murder, and hires Wolfe to exonerate her. Theodore Weed approaches Wolfe, also offering to hire him. He admits that he is in love with Lucy Hazen and that Barry Hazen knew this, taking pleasure from his discomfort about the situation when in her presence. He reveals his suspicions that his employer was extorting money from his clients. Via Nathaniel Parker, Wolfe’s attorney, Lucy gives Wolfe a key to her apartment, and informs him that her husband had given her instructions in the event of his death; she was to locate a metal box hidden in their home, empty the contents, and destroy them.

Archie Goodwin is dispatched to acquire the box, but on arriving at the Hazen residence discovers that the guests from the dinner party are already there, clearly searching the apartment. He manages to hold them at gunpoint, and – after locating the box – brings them to Wolfe’s brownstone. The guests confirm that Hazen was blackmailing them, and inform Wolfe that he took sadistic pleasure in taunting each person with hints about what they had done. Wolfe and Archie open the box only to discover it is empty, but Wolfe nevertheless claims to each guest that he will sell them the contents of the box for $250,000 each.

Inspector Cramer arrives at the brownstone in a gloating mood, revealing that the police have discovered the gun that Lucy Hazen apparently used to murder her husband – a pistol that her father used to commit suicide. The gun is of the same make as the one Lucy brought to Wolfe, however, and the bullet from the first gun did not match the bullet that killed Barry Hazen. This leads Wolfe to a conclusion, which is further confirmed that evening when, alone of the others, Jules Khoury refuses to give Wolfe any money for the contents of the box.

Wolfe reveals that the box was empty and accuses Khoury of murdering Barry Hazen. He admits that he has no evidence, but argues that Hazen’s hints and the specific gun used strongly imply that Khoury’s secret was that he actually murdered Lucy’s father, his former business partner. Furthermore, Khoury’s refusal to pay Wolfe suggest that he knew all along that the box was empty, having located and destroyed the evidence after murdering Hazen. His use of the duplicate gun was an attempt to frame Lucy for the crime. Khoury is arrested and evidence is discovered tying him to both murders, and Lucy and Theodore admit their feelings for each other.

• Nero Wolfe — The private investigator
• Archie Goodwin — Wolfe's assistant (and the narrator of all Wolfe stories)
• Lucy Hazen — Wolfe's client, newly widowed by her husband's murder
• Barry Hazen — Public relations counselor, apparent blackmailer, and murder victim
• Jules Khoury, Mrs. Victor Oliver, Ambrose Perdis, Mrs. Henry Talbot — Clients of Hazen's firm, who pay him much more than his services are worth
• Theodore Weed — Employed as a copy writer by Mr. Hazen
• Inspector Cramer — Representing Manhattan Homicide