The action that made Lance crossed the line (other than his murder) was he manipulated his lover, Gladys Martin, as his cat's paw to murder his father, before he murdered his stepmother and than killed Gladys as well.
In the 1985 BBC Adaptation, he was portrayed by Peter Davison, who is best known for the role of the Fifth Doctor in Doctor Who. In the 2009 adapation from Agatha Christie's Marple, he was portrayed by Rupert Graves.
Eleven years ago, Lance had an argument with his father, Rex, causing a breach, but He began to live in Kenya, and later coming home apparently to reconcile with his father because he is very happy with his wife, Pat, and wanted to settle down. However, he was secretly in another relationship with Gladys Martin, the maid of his father, under the alias of Albert Martin.
In order to obtain the deed of the Blackbird mine, as uranium has been found there, Lance began to manipulate Gladys to kill his father, making her to put the poison in the marmalade (believing it was a truth drug), and the rye in his pocket. The unattractive Gladys was very easy to persuade to assist him, never questioning his motives and flattered by his attentions. He arranged the murder of his father to stop the loss of cash and to deal only with his brother.
Lance later poisoned his stepmother, Adele Fortescue, because she would inherit a large amount of money, but only if she lived thirty days after her husband. After killing his stepmother, Lance lured Gladys out and killed her to silence her, leaving the clothes pin to match the line in the rhyme, "Sing a Song of Sixpence".
In the epoligue, Marple deduced the true identity of the culprit to Inspector Neele. She later received a letter sent by Gladys, who confessed her motives and asked Marple for help. Marple believed that the letter would become an evidence to prove Lance's guilt. Lance was presumably arrested following this.
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