Urim Rama, 27, an Albanian national who moved to London 15 years ago, was on a night out with friends in Chelmsford when he tried to stop a fight in Duke Street.
Mr Rama was with his group of friends when he saw a couple involved in an argument with Jamie Waters in the early hours of August 19.
While Mr Rama tried to calm the situation down, Waters, 29, of Canvey Island, approached Mr Rama and dealt him a blow to his head.
As a result of that one punch, Mr Rama was knocked unconscious and fell to the floor, hitting his head on the pavement, which caused a severe trauma to brain and killed him.
He was taken to hospital and put on a life-support machine, but later died in hospital from his head injuries.
At Chelmsford Crown Court on Wednesday, Waters pleaded not guilty to murder but did plead guilty to manslaughter.
Mr Rama's mother, Bukurije Rama, 57, flew in from Albania especially for the hearing and, flanked by friends and family of Mr Rama, she watched as Waters admitted causing her son's death.
Waters, who was wearing a black suit, sobbed and rubbed his eyes as he pleaded guilty to the charge.
"We still can't believe it. It is not a good result for us because he didn't plead guilty in the right way," said Mrs Rama. "Pleading not guilty to murder was a mistake.
"It's not good for us as he didn't plead guilty in the right way. Not guilty was a mistake. You can't hit someone – if you hit someone and they die and you say you're not guilty, that's wrong.
"You should plead guilty to murder straight away. He should be in prison for life. He should get a life sentence - not three, or five or seven years."
Paying tribute to her son, Mrs Rama said: "He was helping other people – he was protecting an English couple - it wasn't his fault. Wherever he went he helped people. He was like a brother to everyone."
Mr Rama's friend, Nick Kadria, 33, was with him on the night and watched in horror as Mr Rama fell unconscious to the pavement.
"We just can't believe that something like that could happen," said Nick. "We were here to have a good night, to have fun.
"It doesn't matter how drunk he was – Waters threw the punch. Urim kept saying, 'guys just leave it, just leave it', and now our friend isn't alive anymore.
"I thought losing your grandmother was bad but I don't think I will ever get over this."
During the court hearing, prosecuting, Stephen Rose, said: "The Crown have the benefit of CCTV footage which captured from a number of angles the events of that morning. They accept that plea of manslaughter."
When questioned by Judge Christopher Ball QC about the cause of Mr Rama's death, Mr Rose replied: "It's effectively the 'one-punch' manslaughter situation. It was the sort of punch that men use so often in fights that don't cause death.
He added: "There's some evidence to suggest the defendant was pretty drunk when the incident took place."
Defending, Craig Rush said: "It is what it is, I'm afraid. It is what is shown on the CCTV. It's a fact of this case that the defendant accepts he was heavily intoxicated.
Explaining that Waters has got a handful of previous, comparatively minor convictions, Mr Rush continued: "He's not a man of good character but he's not far off it. It's obvious that there will be a custody sentence."
Waters will be sentenced at the end of November.