James McGowan, 58, denies killing Owen Brannigan by repeatedly stabbing him at a house in Coatbridge, on 28 or 29 November 1999.
The High Court in Edinburgh heard recordings of two July 2012 phone calls to a mental health service made by Mr McGowan, who was an expat in Australia.
Mr McGowan told an On the Line adviser he had "crossed a line".
During the phone calls, he told the counsellor he was feeling suicidal. He also spoke of his sons Ryan and Dylan who both played for Hearts.
The jury heard how Mr McGowan, formerly of Coatbridge, told the operator that there was something "slowly eating away" at him.
He added: "Once you've crossed the line and you jump back... you know you can always go across the line.
"Some people can't go across it and I know I went and done it and it's a frightening thing - a frightening thing to deal with.
"You go 'Jesus, I can't kill people. But I did.'"
He also said: "I would have been better off getting caught... you know I probably be coming out to join society again as a better person than what I'm doing - because every single frickin' day it's slowly eating away at me."
The tape recordings were played on the second day of proceedings against Mr McGowan, who was extradited from Australia to Scotland last year.
The court heard Mr McGowan had asked the counsellors not to tape the conversations, but they were recorded in error.
The recordings were later recovered by Police Scotland officers investigating the death of Mr Brannigan.
Mr McGowan and his wife emigrated to Australia in 1980.
Prosecutors claim Mr McGowan murdered Mr Brannigan after he travelled back to Scotland for his mother's funeral in November 1999.
Murray Macara QC, the solicitor advocate representing Mr McGowan has lodged a special defence of incrimination on behalf of his client.
The defence states that a now deceased man called Thomas Stewart was responsible for the alleged murder.
Mr McGowan also denies a charge of assaulting Thomas Duggan at a social club in Coatbridge on 28 November 1999.
The trial, before judge Lady Scott, continues.
The father of former Hearts footballer Ryan McGowan told helpline counsellors he had killed a man, a court has heard.