who is the killer in the book ordeal by innocence
Ordeal by Innocence The killer is revealed to be Kirsten Lindstrom, the Argyles' middle-aged housekeeper. Jacko had persuaded the plain Kirsten that he was in love with her, and persuaded her to murder his adoptive mother under cover of a "foolproof" alibi to steal some much needed money. But once Kirsten learned that Jacko was secretly married, she decided not to cover for Jacko, abandoning him to his fate.
And Then There Were None In the confession, Justice Wargrave writes that he has long wished to set an unsolvable puzzle of murder. His victims would be of his choosing, as they were not found guilty in a trial. He explains how he tricked Dr Armstrong into helping him fake his own death under the pretext that it would help the group identify the killer. He also explains that he replaced the chair in Vera's room. Finally, he reveals how he used the gun and some elastic to ensure his own death matched the account in the guests' diaries. Although he wished to create an unsolvable mystery, he acknowledges in the missive a "pitiful human need" for recognition, hence the confession.