Geoffrey Fardell had been locked inside a jail cell with a repeat offender who has a historical tendency towards unprovoked violence, the NSW Supreme Court was told on Tuesday.

"I'm OK in here mum ... don't worry," Fardell earlier told Sandra Deveson who found this comforting and believed "he would be OK", she said.

Now she has recurring nightmares about him dying alone on the cold prison floor.

Windale man Richard Jason Reay, 46 was found guilty in a judge-alone trial of murdering his 52-year-old cellmate at the Mid North Coast Correction Centre near Kempsey.

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He pleaded not guilty to murder but admitted manslaughter for the June 2019 homicide. However, Justice Robert Allan Hulme threw out his "completely incredible" account saying his version of events "made no sense at all".

Via an audio-visual link Ms Deveson recalled her son's cheeky way of greeting her, with "hello darling," or "mummy," and how he donated to farmers during droughts.

The pair loved catching up for coffee and going to the beach, renovating her house and painting his sister's.

She often thinks of the "frightened terror he must have felt in his last moments of life ...that is now my ongoing nightmare".

It was to her "horror and disbelief," that her son who "loved life and people," had been placed "in a cell with you," she directed towards Reay.

Reay was jailed in 2003 after striking a man with a baseball bat in the head for no apparent reason, and in 2019 alone recorded seven incidents of assault against various prisoners.

While Reay's defence submitted he had accepted responsibility for the killing, despite arguing it was in self-defence, the judge said he would not be allowed much credit for this given his number of lies and lack of remorse.

Reay suffers from a personality disorder and has spent most of his life in some kind of institution, the court was told.

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"The offender has a tendency to engage in unprovoked violence and has done so on many occasions in the past and this is just another instance", the judge said in lieu of having any evidence of a motive.

Reay gave evidence that he had felt "intimidated and vulnerable," by Fardell who he claimed initiated the violent outburst, and in retaliation had strangled him with a clothesline.

The next morning he told a floor sweeper "my celly is dead".

Justice Hulme dismissed his "entirely implausible account of events".

Initially, Reay said he awoke to find Fardell dead in the morning, a story he stuck to until the lesion mark on his neck became apparent.

A registered nurse overseeing medical treatment of Fardell said his behaviour leading up to the incident was not concerning and that "he was lovely".

The Crown submitted Reay had poor or bleak prospects of rehabilitation.

Justice Hulme will deliver his sentence on July 28.

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