The Gravedigger

Heather Taffet aka The Gravedigger was a serial killer who buried people alive.

Victims:
◾Terrance Gilroy (strangled, the first known fatality, the family didn't pay the ransom) - The Boy with the Answer
◾Four unspecified cases of abductions (all paid the ransom and were saved)
◾October 31, 2001: The Kent brothers (both died)
-Ryan Kent (stabbed himself in the neck to give his brother more air)
-Matthew Kent
◾Temperance Brennan and Jack Hodgins - Aliens in a Spaceship (saved)
◾Seeley Booth - The Hero in the Hold (saved)
◾Thomas Vega - The Hero in the Hold (intentionally killed; subdued with the taser and then tasered in the heart)

When Taffett appealed her conviction, it was on her way from prison to the Federal Courthouse to participate in her appeal that she was killed by a single rifle shot to the head. The power of the lone bullet completely destroyed her head and skull, spraying blood and brain matter all over the place, some of which landed on Lance Sweets who Taffett had been taunting or rather mocking just a few seconds before her death. The investigation into her murder formed the basis for the episode The Bullet in the Brain. It was revealed that she was killed by Jacob Broadsky, Booth's former mentor, who had approached James Kent, the father of Ryan and Matthew Kent, the two boys who were Taffett's victims, and offered to kill her for the sum of $2 million; in an ironic twist, this amount was that which Taffet had demanded as ransom for the two boys.

Presumbly, this was an act of revenge on Kent's part as he sought to avenge his two sons.