See How They Run

Leo Köpernick was murdered by Dennis, the theatre usher. Dennis reveals during the climactic confrontation at Agatha Christie's residence that he is the son of a woman whose tragic story inspired Agatha Christie to write The Mousetrap. Feeling that his mother's story was stolen and exploited for profit, Dennis sought revenge against those profiting from the play.

Earlier in the film, Dennis provides vague information about a suspicious figure to the detectives, which hints at his involvement.

Dennis killed Leo Köpernick because he blamed the theatre world and everyone connected to The Mousetrap for exploiting his mother's tragic story. His mother had been the real-life inspiration for the play, and Dennis believed Agatha Christie and those profiting from the adaptation had stolen his family's pain for financial gain.

Dennis also murdered Mervyn Cocker-Norris during a performance of The Mousetrap. After Köpernick's murder, Dennis killed Cocker-Norris to silence him because Cocker-Norris had been conducting extensive research on the play's origins for his screenplay, which threatened to expose the truth about the murder's inspiration.

Mervyn's murder occurs right after Stoppard and Stalker attend The Mousetrap performance, with multiple suspects leaving the auditorium.

Stoppard was arrested because Constable Stalker misinterpreted the scene at The Mousetrap performance. When Mervyn was murdered, Stalker saw Stoppard retrieve his gloves from the floor and exit the auditorium, leading her to believe he was the killer. Combined with the discovery that Stoppard's ex-wife matched the description of a woman in Köpernick's address book, the evidence seemed damning enough to arrest him.

Stoppard's earlier confession to Stalker about his wife leaving him for another man becomes a crucial piece of circumstantial evidence used against him.

Dennis is the son of a woman whose tragic real-life story inspired Agatha Christie to write The Mousetrap. His mother suffered greatly, and Dennis believed Christie and the theatre industry exploited her pain for profit. This personal grievance drove him to kill anyone connected to the play's film adaptation, which he viewed as another exploitation of his family's tragedy.

The connection was a red herring. When Stoppard revealed to Stalker that his wife left him after having a child with another man, Stalker discovered that Köpernick's address book contained the name of a woman matching Stoppard's ex-wife description. This initially cast suspicion on Stoppard. However, Joyce, the mother of Köpernick's son, later confirmed she was not Stoppard's ex-wife, clearing his name.

Stoppard's personal revelation at the pub about his wife's affair becomes the misleading clue that almost incriminates him.

John Woolf was being blackmailed by Leo Köpernick because the director discovered Woolf was having an affair with his assistant, Ann. Köpernick used this information to leverage Woolf into allowing him to direct the film adaptation of The Mousetrap, despite Woolf's initial reservations about bringing on a controversial director.

Joyce, the mother of Köpernick's son, overheard someone speaking with a distinctive "village idiot" accent at the time of the murders. This peculiar detail became a key clue that helped Stoppard identify Dennis as the killer, as Dennis's speech pattern matched this description when he was finally confronted.

At Agatha Christie's residence in Wallingford, Dennis holds Stoppard, Sim, Woolf, Ann, Spencer, and her mother at gunpoint. He reveals his backstory and motive before a chaotic showdown ensues. The stage transforms into a battleground with deception, a poisoned cup, mistaken identities, and life-threatening heroics. Stalker risks her life to stop Dennis while Stoppard confronts him, resulting in Stoppard being injured but ultimately triumphant.

The altercation began when Köpernick, known for his disreputable behavior, confronted lead actress Sheila Sim backstage. The confrontation escalated into a heated argument, which drew the attention of Sheila's husband, Richard Attenborough. The situation deteriorated into a physical brawl involving all three, occurring right before Köpernick's murder and making everyone in the theatre a suspect.