When Colin Howell originally admitted to church elders -- as part of a religious confession -- to gassing his wife and Trevor 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 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.
Defence lawyer Paul Ramsey rejected this explanation of the contradictions in his accounts.
"There was no plan," he insisted. "This was just a spontaneous matter."
Earlier, Howell revealed that the previous night back in Maghaberry prison he had remembered a second meeting with Ms Stewart to plan the murders.
He had originally claimed there was only one face-to-face discussion -- in her car on a roadside lay-by.
But he told the court that meeting was in fact the second of two, with the first having taken place at Ms Stewart's house a few days earlier.
The trial continues.