When mill worker Miss Agnes Montgomery, 27, rejected John Thomsons amorous advances he swore to get even with her. Next time he visited her at her home in Eaglesham, Renfrewshire, he slipped prussic acid into her drink. Within minutes Agnes was writhing in agony, and as she fell to the floor dying Thomson fled.
He went next to Glasgow where he visited a couple who were old friends. While he was having a drink with them he slipped some more prussic acid into their whiskies, intending to rob them when they were dead. But they survived and Thomson was arrested.
Meanwhile, Agnes Montgomery, whose death had been certified as from natural causes, was exhumed and her body was found to be riddled with prussic acid.
At his trial at Glasgow Circuit Court, Thomson denied having poisoned anyone, but the jury found him guilty after deliberating for only 10 minutes. He was hanged in freezing weather on Thursday, January 14th, 1858, outside County Hall, Paisley, before a crowd of 20,000.
Thomson was a career criminal he killed Agnes Montgomery after returning from a transportation sentence for robbery. In the death cell he confessed to the poisonings, and added that when he was only nine or 10 years old he murdered another boy by pushing him into a quarry.