Richard Brady, 33, accused Alistair Balfour, 68, of tampering with his car in a row in the Montgomerie Arms pub, in East Kilbride, on 8 June last year.
He hit Mr Balfour with a single punch, causing him to fall backwards and hit his head on the floor. Mr Balfour died after suffering two skull fractures.
Brady admitted a charge of culpable homicide. Sentence was deferred.
The High Court in Glasgow heard that Mr Balfour was married for 47 years and had an extra-marital affair with Brady's mother which ended in bad terms on Christmas Day 2012.
As a result of this, Brady and Mr Balfour fell out.
They reported each other to the police for vandalism and breach of the peace, but no action was taken because of insufficient evidence.
The court heard that Brady, who is from East Kilbride, entered the pub and accused Mr Balfour of putting sugar in the tank of his car.
He became aggressive and asked if they would take the matter outside.
As Mr Balfour put his hands in his pockets, Brady hit him with a single punch. He fell backwards and lay motionless on the floor.
The court heard that Mr Balfour suffered two fractures to his skull, bruising to his mouth and two black eyes. The cause of death was a head injury.
Judge Lord Turnbull deferred sentence on Brady until next month.

Police also believe the house in which Owen Creaney was allegedly beaten to death was cleaned by the time searches were carried out, prosecutors said.
Mr Creaney, 40, from Lurgan was found dead at Moyraverty Court in Craigavon.
Details emerged as bail was refused to a woman jointly accused of carrying out the killing last weekend.
Shauneen Boyle, 23, of Edenderry Park, Banbridge, denies a charge of murdering Mr Creaney sometime between 4 and 5 July.
Mr Creaney's body was discovered on Saturday in a bin outside the home of Ms Boyle's co-accused, 27-year-old Stephen Hughes.
Post-mortem examinations revealed Mr Creaney, described as frail and vulnerable, died from severe chest injuries and possible head injuries.
Ms Boyle and Mr Hughes were both interviewed more than 20 times and each blamed the other for the killing, the court heard.
Prosecuting counsel said police went to the Moyraverty Court address and asked permission to search it.
Officers spotted blood on the walls and believed the property had been recently cleaned, she said.
It was alleged that Ms Boyle later informed police they should look in the bin where the body was ultimately found.
Following her arrest she claimed to have provided care and assistance to Mr Creaney by visiting him and offering water, the court heard.
But the prosecutor said no professional medical assistance was sought.
She revealed that a witness in the investigation has claimed Ms Boyle confessed to have "gone mad" by punching and kicking the victim.
Questioned by the judge about the state of forensic evidence, the barrister claimed it was limited because Mr Creaney had been showered at the time the injuries were inflicted.
The court also heard the victim may have lay in bed for up to 48 hours after the alleged attack.
Refusing bail, Lord Chief Justice Sir Declan Morgan said: "There's a prima facie case in relation to her involvement in this crime, and there's clear and demonstrable evidence of a risk of violent activities if she were to be released from custody."
A man has admitted killing a pensioner who had been involved in a three-year affair with his mother.
[NEXT_CONCEPT]
A murder victim found dumped in a wheelie bin was showered after being subjected to a violent assault, the High Court has been told.