Richard Jason Reay, 47, was found guilty in a judge-alone trial of strangling Geoffrey Fardell at the Mid North Coast Correction Centre near Kempsey in June 2019.

Justice Robert Allan Hulme jailed Reay for at least 22 years and six months in the NSW Supreme Court on Wednesday, saying he had "absolutely no respect for the sanctity of human life to kill a person in such a callous and sadistic way."

Reay originally denied any involvement saying he awoke to find the 52-year-old lying on the floor, telling a floor sweeper "my celly is dead".

But after a lesion mark was detected on Fardell's neck he pleaded guilty to manslaughter, claiming a story of excessive self-defence over a television volume dispute.

Reay testified that Fardell menacingly ordered "turn the TV down" before leaping on him and delivering "a flurry of punches," causing him to rip down a clothesline that he'd strung above his bed, wrapping it around his neck.

One of Fardell's nurses said his behaviour before had been completely unremarkable while another, asked if she had noticed anything concerning, responded: "No he was lovely".

An autopsy found Fardell was covered in injuries and abrasions including lacerations to the face, Reay in contrast suffered not one, a policeman testified in court.

Justice Hulme threw out Reay's "entirely implausible account of events" saying his story "makes no sense at all," and found him guilty of murder.

Earlier the court heard from Fardell's mother Sandra Deveson who said her son loved life and people and had been looking forward to well-made plans upon release.

But after her son reassured her that he was safe in prison, Ms Deveson now has recurring nightmares about him dying alone on the cold prison floor.

"I screamed out and kept repeating over and over, 'I just want to talk to you, I love you Geoffrey'," she said.

Reay has been diagnosed with severe antisocial personality disorder, and his "long demonstrated propensity for unprovoked and indiscriminate violence," was noted in court.

In 2003 he was jailed for striking a man's head with a baseball bat for no apparent reason, telling a bystander that if weren't for her intervention it was likely to be fatal.

In 2019 alone he recorded seven incidents of assault against various prisoners and has since been transferred to a high-risk maximum security prison

Both inmates were coincidentally transferred to that prison and how they came to be locked in together remains unknown, while Reay's defence implied corrective services were to blame for this placement.

"It is the criminal law that is responsible for inmates being held in correctional facilities, and it is the criminal law that should serve to protect them as far as it can," Justice Hulme said.

"There is little an inmate can do to fend off, let alone escape, from an attack."

The judge described it as "an offence involving an unexplained but extreme act of the most brutal violence directed to ending a man's life," and said he could be afforded no mitigating factors.

"(He) is not genuinely remorseful, he does not have good prospects of rehabilitation, and he does not have an unlikelihood of re-offending," he said.

Reay will be first eligible for parole in November 2042.

Australian Associated Press