The parents of Thomas Kelly, the 18-year-old who was fatally king-hit during a night out in Sydney's Kings Cross last year, have described their son's killer as a coward.

Ralph and Kathy Kelly stood up in the New South Wales Supreme Court on Friday morning to read victim impact statements at a sentencing hearing for Kieran Loveridge.

Loveridge pleaded guilty to manslaughter in June, after the charge was downgraded from murder.

He randomly punched Mr Kelly in the head in July 2012, knocking his victim to the ground and causing catastrophic brain injuries.

The 18-year-old was taken to St Vincent's Hospital, but his life support was switched off two days later.

Loveridge says in days following the assault he could not remember it and he has been troubled by thoughts of the death ever since.

There were tears around the courtroom today as Mr Kelly's parents spoke about the impact of Thomas's death, along with his siblings and two friends.

Among those seen with tears in their eyes were a security guard and Loveridge himself.

Sorry, this video has expired Thomas Kelly's parents tell of grief at king-hit death ( Karl Hoerr )

Kieran Loveridge arrives at the NSW Supreme Court for today's hearing. ( ABC News )

Ralph Kelly told the court of the "intense pain" he still felt and said he would do anything to swap places with his son.

"He never should have died from such a meaningless act of cowardice," Mr Kelly said.

"As a father I should have protected him."

Mrs Kelly told the hearing she thought of her son endlessly and could not understand why he was gone.

"Kieran Loveridge came out of the darkness, like a coward," she said.

"Sydney is becoming a place without respect and without decency at night."

Thomas Kelly's sister told the court she was at university and sometimes when someone mentions her brother's name she would have to leave the room and miss classes for the rest of the day.

Mr Kelly's siblings cannot be named because they are under 18.

Loveridge will be sentenced at a later date for manslaughter and four other other assaults he committed on the same night.

The court on Friday heard one of the other assaults occurred only about 40 seconds before Loveridge punched Thomas Kelly.

Loveridge said in the days following the punch he saw coverage on television and told friends it could have been him but he says he was simply too drunk and did not know.

A psychologist's report has found he is remorseful and has volunteered for alcohol and violent offender treatment while in custody.

The court was told he is troubled by thoughts of the death and is unable to make sense of it himself.

"It's the worst, it's hard, someone died. When I see it on TV I feel sorry for the family. He did nothing wrong. My stupidity caused him to lose his life," Loveridge said.

The court also heard references to Loveridge's difficult childhood and good character.

He was also described as someone who would go too far in trying to be one of the boys.

But parents of his friends describe him as a happy and respectful young man.

He has a past record of assaults while drunk and was on a good behaviour order when he attacked Mr Kelly and four others.

Prosecutors say it shows a pattern of disrespect for the law and other people.

The Crown Prosecutor wants Justice Stephen Campbell not to take too much account of his drunkenness.

"While intoxicated, he still knew enough to act deliberately, intentionally and aggressively to strangers," the Crown Prosecutor said.

When Loveridge pleaded guilty in June, Mrs Kelly said her family had no rights in the decision to downgrade the charge, and warned that violent offenders will "remorselessly reoffend over and over again".

Before Friday's hearing a friend of the Kelly family, David Anstee, said they want people held accountable for their actions.

"Ralph and Kathy want to ensure that the loss of Thomas is not in vain and that the loss of other children like him each and every week in Australia is averted," Mr Anstee told The World Today.

"And that the alcohol-fuelled violence that's besetting our communities and our youth is addressed by regulators, by the NSW Government and by our councils to ensure that our streets are safer and that our [children] can go out at night without fear of harm and to return home safely, unlike Thomas and many others."