Arnfinn Nesset was born on October 25, 1936, in Trøndelag, Norway. He was born out of wedlock and raised by his mother. Nesset lived with her throughout his childhood and into adulthood at her childhood home. His father was not a part of his life and never contacted him.

Nesset pursued an education in nursing. He became a registered nurse. In 1977, he was hired as the head nurse at a larger nursing home located in Orkdal, Sør-Trøndelag.

During the summer and autumn of 1981, many patients died unexpectedly at the Orkdal nursing home in Norway, where Arnfinn Nesset worked as a manager. The police started an investigation into these suspicious deaths. Nesset was questioned by the police and initially confessed to killing 27 patients. He claimed he used a drug called suxamethonium chloride, which paralyzes muscles, to commit these murders. Over time, he was charged with 25 counts of homicide. However, he later retracted his confession and denied all these accusations throughout his five-month trial.

In March 1983, Nesset was found guilty of poisoning 22 patients. He was also convicted of one count of attempted murder but was acquitted on two other counts. Some estimates suggest that he might have actually killed as many as 138 patients during his time at the nursing home.

Nesset received a sentence of 21 years in prison, which was the maximum term allowed by Norwegian law at the time. After serving 12 years, he was released for good behavior. Following his release, he was under 10 years of supervision. It is believed that he is now living in an undisclosed location using a new name. The chief prosecutor at his trial described Nesset as a man who sought complete control over the life and death of his patients.