Lady Westholme

Lady Westholme, also known as Dame Celia Westholme in Agatha Christie's Poirot, is the main antagonist of Agatha Christie's Appointment With Death featuring Hercule Poirot. She is the murderer of the tyrannical Lady Boynton, committing the murder in order to keep her true identity as a former criminal secret.

History
Lady Westholme is a former criminal once imprisoned in the very same prison where Lady Boynton, the book's victim, served as its former prison warden. She later became Lady Boynton's secretary and attempted to reach for higher social status, but was threatened by Lady Boynton, who attempted to leek her secret identity if she did not submit to her. The threat made by Lady Boynton, detailing how she would never forget anything, was originally thought to be a threat to Sarah King, but it was revealed to be a threat to Lady Westholme instead.

Without any choice, Lady Westholme killed Lady Boynton to keep her social status unharmed. Disguised as an Arab servant, she had committed the murder via poisoning the victim with a syringe filled digitoxin, and then relied upon the suggestibility of Miss Pierce to lay two pieces of misdirection that had concealed her role in the murder.

During Poirot's revelation of the truth, Lady Westholme eavesdropped all of it in an adjoining room. Realizing that her criminal history is about to be revealed to the world, Lady Westholme committed suicide.

Trivia

 * In the 1945 stage play version of the novel, also written by Agatha Christie, Lady Westholme had become a purely comic relief character as in this version, Lady Boynton took her own life to cause posthumous pressure and control over her children.
 * Her actress in the 1988 film adaptation, the late Lauren Bacall, had previously portrayed Linda Arden in the 1974 film adaptation of Murder on the Orient Express.