Colin Howell claimed police may have caught him if certain forensic tests were carried out on the body of his second victim, Trevor Buchanan.
The killer dentist, who told Coleraine Crown Court he had studied forensic science as part of an anatomy degree, said he feared detectives were on to him when one used the phrase "perfect murder" when they returned to interview him for a second time about the May 1991 deaths.
But the Royal Ulster Constabulary concluded that Mr Buchanan, the husband of Howell's lover Hazel Stewart, and his wife Lesley Howell had killed themselves in a bizarre suicide pact.
It was only 18 years later when the occasional Christian preacher confessed his crimes that the truth emerged.
During cross-examination by Stewart's defence team as he gave evidence in her trial over the double murder, Howell said histology tests on wounds on Mr Buchanan's body - sustained as he battled for his life with Howell - would have found he died four hours earlier than thought.
The dentist said when detectives returned to question him again two months after the deaths he thought they had made a forensic discovery.
"At that interview I was very conscious if I changed my story that might have betrayed what happened," he said.
"Somewhere in the middle of the interview detective (Jack) Hutchinson said to me, and I don't know why he said it, it would need to be the perfect murder to get away with something like that.
"And that made me wonder if something forensically had been detected."
Howell admitted he used his medical background when executing the plan.
"During my anatomy degree I spent time in the forensic science lab at Queen's University, Belfast, so I picked up things there. So I had an interest in it."
One of the church elders to whom Howell confessed two years ago had told the trial that he seemed to be proud of the fact he had hoodwinked police for so long.
The dentist strenuously denied he was boasting.
"There was absolutely no element of that," he said.
"I am ashamed I used my ability to have ingenuity and intelligence in the wrong way."