Anne Perry killed her girlfriends mother.

   

Anne Perry, the English author of historical detective fiction, best known for her Thomas Pitt and William Monk series, was convicted in 1954 in the murder of her friend's mother, Honora Rieper. Perry, whose real name is Juliet Hulme, was 15 at the time. She changed her name after serving a five-year sentence for Rieper's murder.

The Parker–Hulme murder case took place in the city of Christchurch, South Island, New Zealand, on June 22, 1954, when Honorah Rieper was killed by her teenage daughter, Pauline Parker, and Perry.  The murder has inspired plays, novels, non-fiction books, and several films.

As their friendship developed, Parker and Hulme formed an elaborate fantasy life together. They wrote plays and books and also invented their own personal religion, with their own ideas on morality. They rejected Christianity and worshipped their own saints, envisioning a parallel dimension called The Fourth World, essentially, their version of Heaven

They became obsessed with one another, to the point that Parker's parents became concerned that the girls were engaged in a sexual relationship. (Considered a serious mental illness at the time)

The parents eventually separated them so one afternoon Parker and Hulme went for a walk with Parker's mother and bludgeoned her to death with half of a brick enclosed in an old stocking.

The girls, covered in blood, fled but were met by Agnes and Kenneth Ritchie, owners of the tea shop, and told them that Rieper had fallen and hit her head. Of course the plan failed, the girls were tried in a sensational court drama filled with the speculation about the girls' possible lesbianism and insanity. They were both convicted, and too young to be considered for the death penalty were jailed but only for five years before they were released.