A jury which will decide whether a former Sunday school teacher is guilty of a double murder must keep an open mind, the judge said tonight.
Defence barrister Paul Ramsey QC warned against a miscarriage of justice as the trial of Hazel Stewart (aged 48) accused of killing her husband and her lover's wife, drew to an end.
The jury of nine men and three women at Coleraine Crown Court are due to consider their verdict tomorrow.
Mr Justice Anthony Hart said: "Don't arrive with your minds made up already. I am sure you won't do that."
Prosecutors have insisted the killings of their spouses was a joint enterprise, a plan executed by dentist Colin Howell and Stewart to rid themselves of their respective partners for them to be together.
The judge stressed that the legal definition of a joint enterprise between Stewart and Howell did not mean that Stewart had to commit the murders, only that she was part of the plan to carry them out.
The judge added: "The word plan does not mean there had to be formal agreement about what's to be done, a plan could be made on the spur of moment, with a nod, wink or knowing look.
"Put simply, the question for you is were they in it together?"
Stewart, from Ballystrone Road, Macosquin, Coleraine, who has denied the two murders, sat impassively in the dock today.
Her husband, Pc Trevor Buchanan (aged 32) and Lesley Howell (aged 31) the wife of her lover, were found dead in a car filled with carbon monoxide fumes in a garage behind a row of houses known as the Twelve Apostles in the seaside town of Castlerock, Co Derry, in May 1991.
At first, police thought they had died in some sort of suicide pact because of the distress over their spouses' affair.
It was only when Howell (aged 51) first confessed to his church elders and then police in January 2009 that he had murdered them that Stewart was arrested by investigating detectives.
Howell, a father of ten from Glebe Road, Castlerock, is serving a 21-year jail sentence after pleading guilty to the murders.
He first gassed his wife as she slept on the sofa of their home in Coleraine.
He then drove her body to the far side of the town, where he murdered Pc Buchanan by the same method before taking the two bodies away to stage-manage the suicide.
Mr Ramsey drew the jury's attention to a letter from Howell which he said showed the murder was a spontaneous act by the dentist and that Stewart was surprised to see him show up at her door on the night of the murder.
"It is very revelatory, this letter. This is jaw-dropping," he said.
Mr Ramsey acknowledged that Stewart had failings, was flawed, but it was not her fault the prosecution had not charged her with what she had done, which was assisting an offender and withholding information.
"This is a difficult, distressing and heart-rending case. We have listened day after day to horrific, almost unbelievable evidence as to what happened on that night in 1991," he said.
"It is a tragedy - but we must not compound that tragedy by a miscarriage of justice.
"I would ask you to consider the evidence in this case should give you cause to hesitate, to pause and to resist the headlong urge to convict that the Crown have urged upon you."
He said Stewart was rightfully fearful for herself and her children when Howell arrived at her house to kill her husband.
"In Colin Howell's world there are no rules, no parameters, no boundaries and he is just going to do what he wants to do, and nothing and nobody is going to stop him."
According to the prosecution, Stewart took actions, including making sure no car was in the garage when Howell pulled up at the house with his wife's body in the boot, ensuring her husband was drugged asleep, setting out clothes to dress his body in and afterwards changing the sheets on which Mr Buchanan had struggled for life and burning the rubber hose used to carry in the noxious fumes.
But Mr Ramsey said they were the actions of a woman caught up in an event which she was powerless to prevent.
He added: "With good reason she is scared because he (Howell) crossed the Rubicon.
"He was on a mission and it did not really matter what she did or did not do because he was going to go ahead, so her fears, we say, were quite justified."
Mr Ramsey said it was to Stewart's shame that she did not do anything.
"But how could she without endangering her life or those of her children. Is that unreasonable, members of the jury?
"He is just going to do what he is doing, and nothing or no-one is going to stop him," he added.
He referred to Howell's drugging of Stewart for sex following the murders.
"There is no doubt this sexual relationship was control he was engaged in, not just for his own sexual gratification but using and abusing her as a guinea pig for future 'experiments'," Mr Ramsey added.
He has admitted three charges of indecently assaulting three women on dates between 1998 and 2008. But he has denied a series of other indecent charges involving other women.
Mr Ramsey added: "This is a secretive, clandestine relationship of snatched meetings, weekends away at different hotels, the 'adventures' in surgery were drug-induced and resulted on one occasion in her trying to throw herself out of the window.
"It is two people trapped in a relationship because of the dark secret that they share."
He said if Howell had not lost money on a risky investment in Manila, he would not have confessed.
"Members of the jury, it is all about the money from first to last."