Alleged Sydney jail killer took ice, Xanax

A prisoner with late-onset schizophrenia consumed methamphetamine and five Xanax tablets in the hours before he allegedly beat and strangled a fellow inmate to death in a Sydney jail, a court has heard.

Mervyn Davidson is accused of murdering 54-year-old Alfredo Pengue at Silverwater's Metropolitan Remand and Reception Centre in February 2018.

Davidson was at the facility awaiting sentence after pleading guilty to the drug-induced January 2017 bashing of an Illawarra Aldi cashier with a baseball bat and the subsequent choking of a cellmate with his shirt.

He was sentenced to a maximum of 11 years in prison for the offences.

The Crown stated in the NSW Supreme Court on Tuesday that Davidson and Pengue were in the same pod at the facility and had adjoining cells.

Davidson allegedly stood near Pengue's cell door on the day of the alleged murder shouting: "I will kill all of you."

He went into Pengue's cell for about 25 minutes and left, but then returned, leaving the door open.

Authorities later found Pengue face down with ligature marks on his neck and numerous broken ribs. He was taken to hospital but died.

Crown prosecutor Paul Lynch argued there was "strong circumstantial evidence" against Davidson as he was the only other person in the cell at the time.

Psychiatrist Olav Nielssen, who assessed Davidson for a medical report filed in October 2018, said the accused had a lifelong history of alcohol, cannabis and opioid abuse, but had started hearing voices - including from a woman he called "Senorita" - after taking up methamphetamines.

As a result, he suffered from restricted emotions, blunted thinking and reduced cognitive function, and could feasibly raise mental illness as a defence.

Davidson - 44 years old at the time of alleged offending - had been placed on medication after his first episodes in 2017, Dr Nielssen said.

In retrospect "the dose of medication was too low to adequately control his symptoms".

Davidson also admitted to consuming ice and five Xanax anti-anxiety pills at 8.30am on February 9, some six hours before Pengue's death.

Forensic psychologist Adam Martin told the court those substances may have aggravated the symptoms of schizophrenia by acting as inhibitors, provoking fresh mental hallucinations and hampering memory.

Davidson can't remember the alleged attack.

"It could have led him to act erratically while intoxicated," Dr Martin said.

The psychologist said he was "sitting on the fence" when asked if Davidson's mental illness or substance abuse were the key factor in his alleged crime.

The judge-only trial before Justice David Davies continues.

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