Solicitor and poisoner Herbert Armstrong
1921: Agatha Christie’s first detective novel, The Mysterious Affair at Styles, went on sale in Britain at the beginning of February. While readers puzzled over the identity of the poisoner in rural Essex, a second poisoner was active on the other side of the country, only this one wasn’t fictional.
Herbert Armstrong, a solicitor, thought he could resolve his personal and business problems by getting rid of his overbearing wife and an overly successful professional rival.
Katherine Armstrong died in agony at the end of February from what was initially thought to be gastritis. A few months later, the solicitor Oswald Martin became violently ill after taking tea with Armstrong, but survived (“Excuse fingers,” said Armstrong, as he passed Martin a poisoned scone).
Hercule Poirot was not on hand to solve these cases, but with the help of the forensic pathologist Bernard Spilsbury, arsenical poisoning was shown to have taken place, the arsenic was linked to Armstrong, and Armstrong was convicted of murder and executed.
Source: Colin Evans, The Father of Forensics: How Sir Bernard Spilsbury Invented Modern CSI (2009), pp. 98–118