Serial murderer Steven Grieveson has confessed to also killing a 14-year-old boy found strangled and battered 23 years ago.
Grieveson, 42, admits killing schoolboy Simon Martin at a derelict house in Sunderland in 1990 but denies murdering him, claiming diminished responsibility.
Prosecutors claim he strangled Simon with his hands and a ligature and smashed his skull with a rock.
A jury at Newcastle Crown Court was told he was convicted in 1996 of murdering Thomas Kelly, 18, David Hanson and David Grieff, both 15, in 1993 and 1994.
They had been strangled and their bodies burned, in Sunderland.
In January last year Grieveson told police in an interview he had gone to Gilside House, Roker, with Simon after they had been playing football together in May 1990.
Then aged 19, Grieveson said he and the 14-year-old “performed a sex act” and he became scared his sexuality would be revealed.
He told police: “After I was finished I got scared, I started shouting at him not to tell anyone.
“I just flipped for a minute and I started strangling him.
“I didn’t let go and the next thing, he was on the bed. I think there was a rock or something and I smashed his head in.”
William Lowe QC, prosecuting, told the jury Grieveson took the boy’s trousers and bottoms, throwing his footwear into the sea and binning the clothes.
Grieveson, formerly of Roker Avenue, Sunderland, later told police he had pleaded with his victim not to tell anyone about their sexual contact.
He said: “He was saying he wouldn’t tell but for some reason I didn’t believe him.”
Grieveson told police why he was confessing to killing the teenager, the jury heard.
“I needed to tell police, it has haunted us for 20 years,” he said.
“I have self-harmed because of it.
“It has driven me crazy and I need to give the family peace of mind, myself as well.”
Mr Lowe added: “For whatever reason he strangled with his bare hands and with a ligature this 14-year-old boy and then smashed his skull.
“We said he did that with an intention to kill and that he is guilty of murder.”
The prosecution claimed the violence he used on Simon, and then on his three later victims, may have been to keep them quiet after sex, or to force them into engaging in homosexual acts.
On the evening of May 18, 1990 when he was murdered, Simon had been playing with friends at a local park and his mother Jean had asked him to be home for 6pm in time for his tea, Mr Lowe said.
The police were called at 8pm when he had not returned, and his father, a former soldier, and older brother went out looking for him.
His body was found on May 26 by two boys who were playing in the derelict house.
Simon, who was 5ft 2in and around eight stone, was naked from the waist down and lying on a mattress.
Blood was splattered on the walls and he had suffered a serious head injury.
Semen linked to Grieveson by DNA testing in 2000 was found on the boy’s body.
Bruising to his neck was also discovered, indicating he had been strangled.
The jury was told the defence will claim diminished responsibility and suggest the defendant “by reason of an abnormality of mind” was unable to exercise self-control.
But Mr Lowe rejected that, saying: “Steven Grieveson had the ability to choose whether to kill him or let him go.”
Grieveson denies murder.
The trial continues at Newcastle Crown Court.