Kristen Heather Gilbert was born Kristen Heather Strickland on November 13, 1967, in Fall River, Massachusetts. She was the oldest child of Richard and Claudia Strickland. Richard worked as an electronics executive, while Claudia was a homemaker who also taught part-time. Gilbert had one younger sister.

As Kristen moved into her teenage years, people around her began to notice that she often lied. She also had a troubling history of faking suicide attempts, which seemed to be a way to manipulate those around her. Reports indicate that she made violent threats against others since she was a teenager.

Gilbert graduated from Groton-Dunstable Regional High School in Groton, Massachusetts. In 1986, she enrolled at Bridgewater State College in Bridgewater, Massachusetts. However, her time there was disrupted by a fake suicide attempt, which resulted in her being ordered to receive psychiatric treatment by college officials. Following this event, she transferred to Mount Wachusett Community College in Gardner, Massachusetts, and then to Greenfield Community College in Greenfield, Massachusetts. She successfully graduated from Greenfield Community College with a nursing diploma in 1988, which allowed her to become a registered nurse. Later that year, she married Glenn Gilbert.

In 1989, Kristen Gilbert began working as a nurse at the Veterans Affairs Medical Center in Northampton, Massachusetts. Over time, other nurses noticed an unusually high number of patient deaths during her shifts, leading them to nickname her "The Angel of Death." In 1996, three nurses raised concerns about an increase in cardiac arrests and a decrease in the hospital's supply of epinephrine, a medication she had access to. This prompted an investigation.

As the investigation began, Gilbert attempted to derail it by calling in a bomb threat. After the investigation, she left the hospital that same year. After her departure, she checked herself into psychiatric hospitals multiple times. In January 1998, she went to trial for the bomb threat and was convicted in April.

Due to the suspicious circumstances surrounding numerous patient deaths, prosecutors speculated that Gilbert might have been involved in many more than officially confirmed. Evidence suggested she created medical emergencies to gain attention, especially from her then-boyfriend, who was a VA police officer. He testified against her, claiming she confessed to at least one murder while hospitalized.

During her trial, Gilbert was accused of various crimes, including using poison to attempt to kill patients and deliberately causing emergencies. Notably, there was an instance where she abandoned a patient in cardiac arrest. She was found guilty of three counts of first-degree murder, one count of second-degree murder, and two counts of attempted murder in March 2001.

Although prosecutors sought the death penalty due to the federal nature of her crimes, the jury recommended a life sentence. On March 27, 2001, the judge sentenced her to four consecutive life terms without the possibility of parole, plus an additional 20 years. She was then transferred to a federal prison in Texas, where she remains. In 2003, she dropped her appeal for a new trial after a Supreme Court ruling that could have allowed a death penalty retrial.