John William Agrue was born on September 11, 1947, in El Paso, Texas.

On July 21, 1966, John William Agrue met his 15-year-old sister-in-law, Suzanne "Susan" Marino, in downtown Joliet, Illinois. He asked her to go to Hammel Woods forest preserve with him. When they arrived, Agrue tried to make romantic advances, but Susan rejected him. In anger, Agrue stabbed her approximately a dozen times, resulting in her death. Later that day, he confessed to the murder to his wife. Initially skeptical, she became convinced when he took her to the crime scene and showed her Susan's purse and shoes.

Agrue's wife encouraged him to turn himself in. Instead, he confessed to a priest at a local church, who informed the authorities. Two days later, Agrue was arrested and admitted to killing Susan because she refused his advances. He was convicted and sentenced to 20 to 50 years in prison, sent to the Joliet Correctional Center.

In December 1975, Agrue was moved to the University of Illinois Hospital to donate a kidney to his sister. He escaped on May 11, 1976, after slipping out of a bathroom window and stealing a car. Agrue traveled using false identities and stole cars until he was caught in Central City, Nebraska, a week later.

In January 1982, Agrue was paroled and moved to Colorado. In late June, he murdered 20-year-old Susan Becker by stabbing her 13 times and discarded her body in Boulder Canyon. A week later, he killed 94-year-old Orma B. Smith in her apartment and disposed of her body in Big Elk Creek Meadows. Six days after that, he tried to abduct a woman at knifepoint on the University of Colorado Boulder campus but was captured immediately.

In March 1983, Agrue was convicted of attempted abduction and received a prison sentence. However, he was returned to Illinois for violating his parole. He remained in prison until 1989. After his release, he lived quietly in Joliet and was not linked to any violent crimes during this time, though some neighbors reported disturbing behavior.

John Agrue died on June 29, 2009, from an accidental overdose of prescription medication. Authorities found women's purses and jewelry in his home along with old newspaper clippings about his crimes. After his death, investigators matched his DNA to the Smith murder scene, officially solving that case. Agrue was also considered a suspect in Becker's murder due to evidence placing him near the scene. In May 2013, he was labeled a serial killer based on the similarities of his known murders, and police began reviewing cold cases for possible connections to him. As of July 2022, none have been linked to Agrue other than the known cases.