John Cooper destroyed the lives of many people, including those of his own family.
The twisted killer murdered siblings Richard and Helen Thomas in cold blood in 1985, then struck again four years later in a second sickening double killing of hikers Peter and Gwenda Dixon.
In both attacks, the evil killer tied up his victims then fired at them with a shotgun after trying to steal money from them.
It wasn’t until decades later, when Cooper was behind bars for a string of violent burglaries, that the police came to a shocking realisation.
They were stunned to discover that darts fanatic Cooper had appeared on ITV gameshow Bullseye exactly a month before killing the Dixons and perfectly matched the artist’s impression of the killer.
While being interviewed by police, Cooper sensed he was under threat and disgustingly tried to implicate his own son.
Detective Superintendent Steve Wilkins, who was the person who decided to reopen the cold cases, was disgusted by Cooper’s repellent stratey.
John’s estranged son Andrew Cooper actually helped write The Pembrokeshire Murders because he had “a score to settle with his dad”.
Show writer Nick Stevens was determined to give Aidan, who now goes by the name of Andrew, a voice in the ITV drama.
ITV journalist Jonathan Hill managed to track Andrew down as they wanted to make sure he was comfortable with his story being told publicly.
“Over the course of several meetings – quietly, frankly and without self-pity – Andrew told me how it had been for him growing up as the abused son of a serial killer,” explained Nick.
“He also breathed life into the tragic figure of his mother, Pat, who, in another extraordinary twist, had died of a heart attack on Cooper’s first night of freedom after his release from prison in Christmas 2008.”
In the second episode of The Pembrokeshire Murders, viewers saw distressed Pat lock herself in the bathroom as soon as her husband got home as a shock cliffhanger.
In real life, Pat did die on the night her husband came home on parole but the police did not find any connection between Cooper and her death.
Pat suffered from several heart problems and it is believed the stress of her husband’s trial may have exacerbated her long-term health issues.
“Cooper came out of prison, moved back in with his wife, and on the first night he moved back into that house his wife died – I just think she gave up,” Wilkins told Wales Online.
“She’d had 10 years away from this man who was an absolute beast and suddenly he was back in the house. She had massive heart conditions but I think the poor lady just gave up.
“There was nothing linking her death to Cooper though. You can imagine – I got the call at 3am from our control room to say John Cooper just phoned us up to say his wife’s dead.”
It was Cooper’s wife, who put up with years of abuse at the hands of her cruel husband, who unknowingly provided the piece of evidence that brought him to justice.
A police sketch from 1989, known as the “wildman” sketch, depicted a man seen using Peter Dixon’s cash card at an ATM after his murder.
A policewoman noticed some khaki shorts seized from Cooper bore a resemblance to those in the sketch, and also appeared to be women’s.
They turned out to be Gwenda Dixon’s, kept as trophy by Cooper, whose unsuspecting wife turned them up – and in doing so trapped vital DNA evidence in the seams.
The DNA evidence that was the “golden nugget” that brought Cooper down, as a microscopic spec of blood and fibres in his shorts matched that of the Dixons.
“It’s absolutely incredible to think Pat’s simple act would lead to him finally facing justice,” said Wilkins.
Cooper was arrested in May 2009 after a lengthy investigation and forensic costs that were more than £1million over-budget.
He was found guilty of the two double murders of Peter Dixon and Gwenda Dixon and Richard Thomas and Helen Thomas after an eight-week trial at Swansea Crown Court in May 2011.
In addition, the jury further convicted Cooper of sticking up five terrified teenagers at gunpoint in woods on Milford Haven’s Mount Estate in 1996, where he raped one 16-year-old girl and indecently assaulted another, aged 15.
Cooper was jailed for life for his sickening crimes, but tried to challenge the conviction in 2012.
The killer had his application for an appeal thrown out by Lord Chief Justice, Lord Judge, sitting at Cardiff Crown Court.
The Lord Chief Justice said: “The defence case was the applicant had been wrongly convicted of the Huntsman offences and that he was not guilty of the present offences.
“The jury no doubt considered his [Cooper’s] evidence carefully before deciding his guilt was proved.
“The application for leave to appeal against conviction was refused by a single judge. We agree with him. This renewed application will similarly be refused.”
He remains locked behind bars to this day at the age of 76.
Casting a real life criminal is tricky, but Keith Allen has been incredible in the role of Cooper.
Keith has delivered an incredibly chilling performance as the murderer, whilst showing different aspects to his character.
“Keith Allen was quite brilliant at absorbing the traits of Cooper and portraying a kind of strange dynamism that the man obviously had,” said director Marc Evans.
While ITV journalist Jonathan Hill added: “Keith absolutely inhabited Cooper.”
Keith, who is dad to famous singer Lily and Game of Thrones star Alfie Allen, was digitally inserted into real footage of the Bullseye episode that Cooper appeared on.
He was also de-aged to bring to life the disturbing scenes from the 80s.
*The Pembrokeshire Murders concludes tonight on ITV at 9pm