Convicted murderer Colin Howell has begun giving evidence at Coleraine Crown Court on Monday in the murder trial of his former lover.
Howell, 51, has already pleaded guilty to killing his wife Lesley and Trevor Buchanan, the policeman husband of his then mistress Hazel Stewart.
Hazel Stewart, 47, denies the 1991 double murder.
Howell explained why he had confessed to the crimes, which he originally staged to look like a suicide pact.
The former dentist, who is serving a 21-year sentence at Maghaberry prison, had poisoned the pair as they slept.
He then tried to make it look as if they had taken their own lives in a suicide pact.
"I just knew that the time had come that the truth had to be told," Howell told the court.
"I was overwhelmed by my conscience from hiding those crimes for such a long time.
"I believed there still were scars that needed to be put right and I wanted to tell the truth, that was my only motive."
The bodies of Mrs Howell, 31, and Mr Buchanan, 32, were found in a fume-filled garage in the seaside town of Castlerock.
Mrs Stewart, from Ballystrone Road, Coleraine, denies entering into a joint-enterprise with Howell to murder their respective partners.
She told police it was his plan and that she wanted nothing to do with it.
Howell recalled in court on Monday how and why his affair with Stewart had developed in 1990.
He claimed he and his wife Lesley had relationship problems before their marriage in 1983 and that these had deteriorated as the years passed.
Howell claimed his wife had started to express doubts about why they had got married in the first place and that he had lost his motivation and "began to look around".
"We came into the marriage with unhappiness between us but we both sincerely wanted it to work, but it didn't," he added.
Howell said his unhappiness grew when his wife was pregnant with their youngest two children Daniel and Jonathan in the late 1980s - a time, he said, when he had to shoulder more responsibility in terms of the family.
"Lesley was very astute and intelligent, a match for me, so there were often arguments and debates and she generally got the upper hand," he said.
He said this had made him feel insecure and he wanted to "look for someone who approved of me".
"That's probably part of the reason for having an affair with Hazel - the attention I got," he said.
Howell said he had got to know Stewart, who also attended his church, when she worked as an assistant in the playgroup his children attended.
He said they grew closer when they both took their children for swimming lessons.
"She approached me and said she was impressed with the way I was doing my front crawl," he said.
"That she would like to do it the way I did because she had problems breathing."
Howell said he gave her a few lessons and on the third he recalled running his hand across her stomach.
"She didn't object," he said.
Howell said he then apologised and admitted he was not having "innocent thoughts" about her.
"She said 'Well, I'm not that innocent myself' and she swam away," he added.
Howell said the affair developed into a sexual relationship weeks later when he went round to her house on the pretence of teaching Stewart the guitar.