Mathew Charles Welch, 47, is accused of repeatedly bashing Peter and Desley Welch, his adoptive parents, both then aged 83, in the early hours of April 6, 2013.

In documents considered by Wollongong District Court on Friday, Welch implied Mrs Welch had slapped him for no reason as a child.

Defence solicitor Nerissa Keay, acting for Welch, said events of the past had motivated her client's attack.

"What's happened to him as a child is what was playing on his mind at the time these offences were committed," she said.

In an attack lasting seven hours, Welch repeatedly kicked and punched both parents; poured a bottle of milk over them; hit his father over the head with a bottle of Cointreau, held a sword to his father, placed a knife under both parents' noses, destroyed the phones, ransacked the house and refused to let his father call the hospital for help.

He also started a fire on the floor between his parents and repeatedly threatened their imminent death but - bizarrely - also covered them with doonas in a possible act of kindness.

A neighbour responded to a cry for help from the house about 6am, arriving to find Peter Welch covered in blood, with extensive head wounds, spine fractures and a serious leg wound.

The neighbour found Mrs Welch unconscious, her face "black and blue". She was left with horrific head injuries requiring a 10-day stay in the intensive care unit.

Welch attended Wollongong Hospital and when told that the injuries to his mother were very serious and she may not survive, replied: "Payback's a bitch."

He has pleaded guilty to two charges of causing grievous bodily harm (with intent).

Judge Paul Conlon will consider documents, including a psychologist's report in which Welch's behaviour is described as consistent with personality disorders including borderline personality disorder.

Elsewhere in the material tendered to the court, Welch expressed remorse at the serious harm he had caused to his adoptive parents.

The matter was adjourned to April 8.