She was born Daisy Hancorn-Smith in 1886 in Seven Fountains in the Cape and trained as a nurse. She married William Cowle, a plumber, in 1909; he died in January 1923 after violent convulsions, and she collected on his insurance. She married Robert Sproat in 1926; he died in November 1927 in the same way, and she collected again. Both deaths were certified as natural, cerebral haemorrhage and chronic nephritis, and strychnine convulsions were mistaken for both.
In 1931 she married Sydney de Melker, a former Springbok rugby player. In March 1932 her twenty-year-old son by her first marriage, Rhodes Cowle, died. He had been difficult, was pressing her about money he believed was owed him from his father's estate, and she had given him arsenic in a flask of coffee.
A relative went to the police, and Rhodes's body was examined. Arsenic was found, and Cowle and Sproat were exhumed and found to contain strychnine. A chemist's assistant identified her as the woman who had bought strychnine, signed for as being for a sick cat.
Her trial in Johannesburg lasted a month and the public queued overnight for seats. She was convicted of the two poisonings of her husbands and of her son's murder, and hanged at Pretoria Central Prison on December 30, 1932. She was the second woman hanged in South Africa.
The case is the foundational South African poisoning case and is still taught there. The detail that has always carried it is the ordinariness: a respectable nurse in a Johannesburg suburb, twice widowed with sympathy, and a son who wanted his inheritance.