The 78-year-old brother of Hollywood actress Deborah Kerr was killed after a motorist lashed out at him during a "road rage" incident in Birmingham, the city's Crown Court heard.
Edmund Trimmer struck his head on the pavement as a result of a powerful punch to his jaw suffered in an unprovoked attack by grandfather Eugene Warwood, who then drove off.
Warwood (56) of Salop Road, Redditch, Worcestershire, had previously admitted the manslaughter of Mr Trimmer, whose sister was nominated for several Oscars for her roles in blockbusters such as From Here to Eternity and The King and I.
Timothy Raggatt QC, prosecuting, said the victim was a married man who lived in Kings Norton and who had a distinguished career with Central TV as a journalist and news broadcaster.
He said on August 23 last year Mr Trimmer had gone out in his Rover to post a letter while Warwood, driving an Escort van, had been to visit his daughter in West Heath.
Mr Raggatt said what happened before the incident was unclear and it was possible the two vehicles had clipped wing mirrors in Middlemore Road, Northfield.
He said Mr Trimmer may have inadvertently parked in a place where Warwood wanted to park, but, for whatever reason, the defendant had become "incensed".
Warwood drove his van beyond Mr Trimmer's vehicle and got out at the same time the victim was emerging from his car.
"There is clear evidence he was being abusive and was shouting and when he got to Mr Trimmer who was by now standing by the door of his vehicle he simply lashed out at him with his fist and struck him on the jaw," said Mr Raggatt.
He said the blow was powerful enough to break the retired journalist's jaw and knock him out.
"Having felled Mr Trimmer in that way Warwood simply turned on his heels and left, getting into his van and driving away," said Mr Raggatt.
He said the victim had shown no hostility or violence and that a number of people saw the attack and were shocked by what happened.
Mr Trimmer was rushed to hospital but died the same day as a result of a serious head injury, caused when he struck the ground.
John Jenkins QC, defending, said Warwood, who had five grand children, had seen red when the vehicle's wing mirrors had clipped and had acted in a "flash of temper".
He said he felt a deep sense of shame and remorse for what he had done and that he had had no idea that Mr Trimmer had been fatally injured.
Judge John Saunders QC adjourned sentence until this morning.