Simon Peter Nelson was born on October 2, 1931, in Rockford, Illinois. He grew up in a difficult home environment. His father was known to have physically abused him during his childhood. In 1954, his father committed suicide, which deeply affected Simon. During his trial years later, Simon claimed that he found his father's journal. In this journal, his father expressed feelings of blame toward the family for his sadness.

On January 7, 1978, in Rockford, Illinois, Simon Peter Nelson killed his six children and the family dog after his wife called to inform him that she wanted a divorce. He attacked the children while they slept, using a knife and a rubber mallet. After committing these acts, he drove to Milwaukee and assaulted his wife, admitting that he had killed the children during the attack. Police arrested him midway through this incident.

During his trial, Nelson claimed he could not remember the killings. His defense team argued that he was not guilty due to insanity. However, the jury found him guilty of six counts of murder. The prosecution sought the death penalty. In the sentencing phase, the jury could not reach a unanimous decision, voting 11-1 in favor of the death sentence. As a result, he received a sentence of 100 to 200 years in prison, with officials recommending he never be released.

Nelson became eligible for parole in 1986 but faced multiple denials. He had 18 parole hearings before his death in 2017. In one of his hearings in 2004, he expressed that he regretted his actions and stated that he had benefited from anger management therapy while in prison. He claimed to remember only fragments of horror related to the crimes. He mentioned feeling his deceased father’s presence during the events.

As a model inmate, he worked as a library clerk starting in the 1980s. In a 2008 parole hearing, he argued that his commitment to his job showed he could be a productive member of society. Despite his claims of reform, members of the parole board consistently denied him release.

In 2014, he repeated his claims about the limited memories of the crimes and stated his faith. Simon Peter Nelson died in prison on June 18, 2017, at the age of 85 while awaiting another parole hearing. Throughout his nearly 40 years in prison, he never received a single vote in favor of his release.