Steven Kirkwood, 44, stabbed Michael O'Hanlon four times during an incident at HK Autotek garage in Stevenston, North Ayrshire, on 25 July 2016.
The 45-year-old died from blood loss.
Kirkwood was originally charged with murder, but the jury at the High Court in Glasgow unanimously convicted him of the reduced charge of culpable homicide under provocation.
The court heard that the killing was sparked by abusive emails sent by Kirkwood and his former partner Eileen O'Hanlon, who was due to marry Mr O'Hanlon.
Judge Lady Rae told Kirkwood it was "frankly astonishing" to see adults behaving in this way.
She said Kirkwood had challenged Mr O'Hanlon to fight, set the place and the time, and had gone armed with a lethal weapon.
She described how the fight had lasted no more that 30 to 60 seconds, in which time Mr O'Hanlon sustained fatal injuries.
Lady Rae said: "You now, I am told, express some regret for the killing, although somewhat belatedly I suspect that regret is directed more towards your predicament rather than real remorse for what you did.
"As you gave evidence I detected no remorse whatsoever for causing the death of another human being, you seemed intent on placing the blame on everyone else rather than yourself."
Outside court Mr O'Hanlon's niece Haylie O'Hanlon said: "Steven Kirkwood is an evil man. I hope he rots in jail for the rest of his life."
The victim's sister Janice O'Hanlon said: "Michael never sent any of those texts. Michael was a big softy. Body-building was his life and if Steven Kirkwood hadn't killed him he would still be competing.
"It chilled me to the bone listening to what he did to Michael."
Mr O'Hanlon's fiancÃ©e Eileen O'Hanlon - who changed her name by deed poll to take his surname - was not in court to see Kirkwood sentenced.
Kirkwood claimed that he had acted in self defence after being assaulted by Mr O'Hanlon, from Irvine, and his friend Forbes Cowan, 52, a former competitor in the World's Strongest Man contest.
His QC Derek Ogg said: "Mr Kirkwood has expressed empathy and remorse."
A man who stabbed a former Mr Scotland bodybuilding champion to death has been jailed for 14-and-a-half years.