Colin Howell, a dentist from Coleraine, N Ireland and his accomplice lover Hazel Stewart. Both now serving life terms.
Howell murdered his wife and Stewart's husband in a staged "suicide pact" that fooled the authorities.
The murders came to light 18 years later only because Howell, a Christian, confided in a church elder.
Stewart had assisted him by drugging her husband and covering up the crime. Her second husband, a police chief, insisted that she was controlled and manipulated by Howell, and tied in vain to get her released.
Another killer who almost got away with it is Texan pastor Matt Baker, who killed his wife Kari by drugging and suffocating her. Baker told police she had killed herself while depressed.
The authorities ruled the death a suicide but her mother said Kari would never have left her young children and hired a private investigator. It emerged that Baker was having an affair with a parisioner, who initially denied any knowledge but eventually told authorities she knew Baker was planning to kill his wife. She was granted immunity and he's serving life.
Both these stories break my heart, as do others mentioned here, especially Breck Bednar. Also Kayleigh Haywood, age 15, who was groomed online by Luke Harlow. A neighbour heard Kayleigh's screams just before she was murdered by Harlow's friend Stephen Beadman but did not call police.