A cop who turns into a killer? Well, it has been known. It happened for instance in June 1893, when a shepherd crossing the common land that in those days was opposite Wormwood Scrubs Prison found the body of a young woman spreadeagled on the grass. She was Maud Merton, 22, a prostitute, and although she had been battered to death there were no signs of sexual assault.
It turned out that Constable George Cook, whose night beat took him past the prison, had been having an affair with Maud for the past two years. A search of his lodgings revealed a bloodstained uniform, and his truncheon, also bloodstained, was in his garden shed.
Cook was brought to trial at the Old Bailey. It was said in his defence that Maud had been stalking him and threatening to expose their relationship when she learned that he had a new girl friend. He was hanged on Tuesday, July 25th, 1893 at Newgate Prison.