Mervyn Keith Davidson was on remand for several violent offences in February 2018, when he walked into 54-year-old Alfredo Pengue's cell at Silverwater prison, shut the door and strangled him.

Supreme Court Justice David Davies on Wednesday said he couldn't be satisfied beyond reasonable doubt the 46-year-old intended to kill Pengue.

But he said the killing met all the elements of manslaughter.

Shortly before the killing, Davidson consumed ice and Xanax and told other prisoners he would "kill them all".

He was calm and composed when interviewed by police about his 25 minutes in Pengue's cell, but refused to answer questions.

"I know what I'm here for. Just hurry up and do the charge paperwork so I can go back to my cell and sleep," he said.

Two medical experts later diagnosed Davidson with schizophrenia, with one suggesting the inmate's long history of ice use may have caused the disorder to previously be labelled as drug-induced psychosis.

He had reported previous occasions of hearing voices emitting from televisions and radio telling him to do bad things, Justice Davies said.

At the time of the killing, Davidson was awaiting sentence for attacking a female Aldi worker with a baseball bat during a botched supermarket robbery in Albion Park Rail in January 2017.

While in custody for that matter, he punched a police officer and choked his Silverwater cellmate with a t-shirt until the man became unconscious.

Davidson, housed in his own cell after that attack, consumed ice weekly in custody in the lead-up to the killing on February 9, 2018.

Shortly after Davidson left Pengue's cell, the victim was found by another inmate on the ground with blood dripping from his mouth and his head almost purple upwards from a horizontal mark on his neck.

Davidson, who is already serving an 11-year jail term, will be sentenced at a later date.

Australian Associated Press