Ronald Joseph DeFeo Jr. was born on September 26, 1951, in Brooklyn, New York. He was the oldest child in the DeFeo family. Ronald had two brothers and two sisters. His father, Ronald DeFeo Sr., was a successful car dealer, while his mother, Louise, was a homemaker. The family moved to Amityville, New York, in 1965, where they purchased a house at 112 Ocean Avenue.

Around 6:30 p.m. on November 13, 1974, Ronald DeFeo Jr. entered Henry's Bar in Amityville, New York. He shouted for help, claiming that his parents had been shot. DeFeo and several people went to his house at 112 Ocean Avenue. Inside the house, they discovered that DeFeo's parents and four siblings were dead. His friend Joe Yeswit called the police, who found all six family members lying face down in their beds.

The victims included DeFeo's parents, Ronald DeFeo Sr. and Louise DeFeo, as well as his siblings: Dawn, Allison, Marc, and John. All had been shot with a .35 caliber rifle. The children had one shot each, while both parents had been shot twice. Evidence suggested that DeFeo's mother and sister Allison were awake at the time of their deaths.

Initially, DeFeo was taken to the police station for his protection. He suggested that a mob hitman named Louis Falini was responsible for the murders. However, his story had inconsistencies, and the next day, he confessed to killing his family. He described how quickly it all happened and told detectives he couldn't stop once he started. He also led the police to where he dumped evidence, including his blood-stained clothes and the rifle.

DeFeo's trial started on October 14, 1975. His defense lawyer argued that he was insane when he committed the murders. However, psychiatric experts disagreed, stating he was aware of his actions. On November 21, 1975, DeFeo was found guilty of six counts of second-degree murder. He was sentenced to six terms of 25 years to life in prison.

While in prison, DeFeo made several claims about the night of the murders. He claimed that Dawn had killed their father and that their distraught mother killed the siblings before he shot his mother. He later changed his story, asserting that only Dawn was killed by him, and he had acted in self-defense. The courts found these claims to be unconvincing, as no evidence supported his assertions about anyone else being involved.

DeFeo continued to seek to have his conviction overturned throughout the years but was unsuccessful. He died on March 12, 2021, at the age of 69 in Albany Medical Center. The cause of his death has not been revealed.