Dentist Colin Howell tried to trick psychiatrists into thinking he killed his wife on the spur of the moment in order to get a shorter sentence, he told the trial of his former lover Hazel Stewart.
The one time lay preacher, who went on to murder Stewart's policeman husband Trevor Buchanan, said he told police his decision to murder his wife Lesley was spontaneous in the hope of convincing medics to support a plea of manslaughter on the grounds of diminished responsibility.
Howell gave this version of events to detectives inside prison two years ago - five months after he had confessed to the May 1991 killings - but he has since pleaded guilty to murder charges and claimed the deaths were extensively planned in conjunction with Stewart.
Giving evidence for a fourth day, Howell also told Coleraine Crown Court he now remembered another planning meeting with Stewart - he originally claimed there had been only one - in the days before the killings.
Stewart, 47, from Ballystrone Road, Coleraine, denies being part of a joint enterprise with Howell to murder their spouses.
Her defence barrister, Paul Ramsey QC, put it to Howell, who has been sentenced to 21 years, there was no plot and that his June 2009 interview with police clearly indicated that.
But the father of 11, dressed in a grey suit, told the jury the account he gave to detectives was manufactured.
"I began to deceive myself and even believed this was a spontaneous act that day. I was beginning to lie again, beginning to see that if I could get it to be believed this was a spontaneous act that day psychiatrists would support a psychiatric plea and I would get less time in prison," he said.
Howell said when he originally admitted to church elders - as part of a religious confession - to gassing his wife and Mr Buchanan and staging it to look like suicide he intended to take responsibility for his actions.
But he said the mental impact of his second wife leaving him upon his imprisonment led him to consider a manslaughter plea.
He said he knew the legal system could be used and abused.
"I was trying to achieve my own ends by saying it was spontaneous," he said.
The dentist explained that he subsequently had his faith in God renewed and decided he must take full responsibility, plead guilty to the murders and admit the crimes were planned.
The born-again killer who referred to the Bible on numerous occasions during his cross examination in the witness box revealed his legal team had urged him "not to preach".
He nevertheless went on to invoke theology again when explaining why he believed he could not accept manslaughter pleas and the promise of a shorter prison term.
"If you confess your sins Jesus is faithful and just and will forgive your sins and cleanse you from all unrighteousness," he said.
Mr Ramsey rejected this explanation of the contradictions in his accounts and claimed he was running away from the truth.
"There was no plan," he insisted. "This was just a spontaneous matter."
Earlier Howell revealed that last night back in Maghaberry prison he had remember a second meeting with Stewart to plan to murders. He had originally claimed there was only face-to-face discussion - in her car on a roadside lay-by.
But he told judge Justice Anthony Hart that meeting was in fact the second of two, with the first having taken place at Stewart's house days earlier.
Stewart, wearing a plum coat, occasionally glanced at her former lover from the dock as his testimony against her approached 15 hours.
The bodies of Mrs Howell, 31, and Mr Buchanan, 32, were found in a fume-filled garage in the seaside town of Castlerock, near Coleraine.
Howell had poisoned them with carbon monoxide piped from his car as they slept in their own homes, then set up a scene to look as though they had taken their own lives because they could not cope with his and Stewart's affair.