Trevor Timon hit Oliver Dearlove while he and his friends were talking to a group of women in Blackheath, south-east London, in August.
The court heard a series of WhatsApp messages sent by Mr Timon where he told a female friend he had "lost it all".
The 31 year old, from Plumstead, admits manslaughter but denies murder.
Mr Dearlove, who lived with his girlfriend Claire Wheatley in New Eltham, died within 24 hours of being attacked.
One of the four women who witnessed the punch told the Old Bailey she had known Mr Timon for 10 years and said he had been left in a "terrible state".
The woman, who cannot be identified, was asked to read a series of messages sent to her by Mr Timon over the days following the fatal attack in which he pleaded for forgiveness.
In one he said: "No words can say how sorry I am. I'm not running away from it."
In another, he wrote: "I'm so devastated about this. I'm in complete shock... All I can do is tell the truth... I'm scared."
"I never ever meant to do it... I'm sorry really from the bottom of my heart... I just wanted to live life and now I feel like I have lost it all," he said.
The woman also read messages she had sent to a female friend hours after the incident where she said "Trevor hit someone for no reason".
She told the jury she was "very upset" with Mr Timon after the attack but claimed she may have "exaggerated" because she was annoyed at him for "spoiling" the night.
The case continues.
A man accused of murdering a banker with a single punch during a night out told a friend he was "devastated" by what happened, a court heard.