One of thirteen children, born and raised in the Midwest, Robert Diaz was frequently ill during childhood, completing only ten years of formal education. He joined the Marine Corps at age eighteen, then went AWOL for six weeks and was discharged soon after his return to duty. Married in Jacksonville, Florida, during 1961, he was divorced in 1972. By that time, Diaz had decided to act on his long-standing desire for a medical career, and after the divorce he enrolled in a school for vocational nurses. At family gatherings thereafter, he insisted that the children introduce him to their friends as ''Dr. Diaz."

By early 1981, Diaz was working his way through a series of temporary nursing jobs in several Southern California hospitals. Coincidentally administrators noted that their older patients had begun to die in record numbers; by the end of April, there were 30 suspicious deaths on file in Los Angeles County, with an equal number spread over Riverside and San Bernadino Counties. Autopsies revealed high levels of lidocaine -- a drug used to control irregular heartbeats -- in most of the corpses. While normal readings should have indicated 50 to 100 milligrams of lidocaine, the recent dead showed levels of 1,000 milligrams and more -- enough to bring on cardiac arrests instead of saving lives.

A search for common links between the victims demonstrated that a single nurse -- Diaz -- had worked at each and every institution where abnormal death rates, were recorded. Attention focused on Diaz after the death of Estel Jones, age 62, at Chino Community Hospital. First ascribed to "natural causes," her death was later ruled a homicide, and officers began examining the high mortality rate on Diaz's graveyard shift. A search of the suspect's home turned up bottles of lidocaine, but charges of illegal drug possession were later dismissed. Diaz fired back with multimillion-dollar lawsuits charging defamation of character and civil rights violations, but skeptical judges dismissed each in turn.

The investigation rolled on, detectives examining fifty deaths, finally narrowing their sights to a dozen where evidence seemed incontrovertible. On November 24, 1981, Robert Diaz was arrested on twelve counts of first-degree murder: eleven victims, ranging in age from 52 to 89, were killed at Community Hospital of the Valleys, in Perris, California, between March 30 and April 22, 1981; the twelfth, a 79-year-old man, died of a lidocaine overdose at San Gorgonio Pass Memorial Hospital, in Banning, on April 25.

Diaz entered his formal plea, innocent on all counts, at the end of September 1982. His trial was scheduled to open in March 1983, but various postponements delayed the event until Halloween. Waiving his right to a jury trial, Diaz placed his fate in the hands of a judge, who convicted him on all counts on March 29, 1984. Two weeks later, on April 11, Diaz was sentenced to die in the gas chamber at San Quentin.