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.
A man has admitted killing a pensioner who had been involved in a three-year affair with his mother.