A Sydney man who gave his seriously ill partner a lethal shot of heroin in what a judge has accepted was a "mercy killing" will spend at least 10 years behind bars.
Robert Ronald Cooper, 47, pleaded guilty to murdering his partner, Donna Green, 55, in Bankstown in early March 2017.
In the NSW Supreme Court on Friday, Acting Justice Peter Hidden sentenced him to 13-and-a-half years in jail with a non-parole period of 10 years.
The court heard Ms Green had been seriously ill and was admitted to hospital 24 times in the 13 months before her death.
On the day she died, Cooper sent a series of texts to family and friends that Ms Green was in pain and "wanted to go away", telling them he had given her heroin.
Police were called to Ms Green's home about 7.30pm on March 5, prompting Cooper to flee. By this time Ms Green had been dead for at least two hours.
Cooper was arrested the next day and charged with murder.
Acting Justice Hidden was satisfied Ms Green asked Cooper to give her a lethal shot of heroin to take the pain away.
"I accept ... this was a mercy killing," he said.
"I accept he did indeed love the deceased ... he made an agonising decision."
The court heard Cooper and Ms Green had been in an "intimate relationship" for two years.
The relationship was described by Acting Justice Hidden as volatile, unstable and domestically violent.
When Ms Green died, Cooper was in breach of an AVO taken out to protect his partner.
Two months before he killed her, Cooper had asked Ms Green for money outside Royal Prince Alfred Hospital. He abused her and threatened to stab her when she refused.
However, despite the pair's turbulent history, Acting Justice Hidden said the killing "could not be seen as an act of domestic violence".
"In the eyes of the offender, what he did was an act of love," he said, after reading out a series of messages Cooper sent to family and friends of Ms Green where he said she "wants to go away".
"I don't care if people say I killed her (or if I go to jail) I'd rather do that than see her in pain," he wrote in one message.
Acting Justice Hidden said the messages depicted a "deeply conflicted man".
But the Supreme Court judge acknowledged that regardless of Cooper's intention he'd robbed Ms Green's family of a woman who was much-loved.
He described a victim impact statement that was a "moving expression of the love (the family) had for the deceased and the depth of grief they feel from her loss".
The court heard that two psychiatrists agreed Cooper suffers from chronic schizophrenia.
That diagnosis, Acting Justice Hidden said, could have amounted to a possible partial defence for Cooper but he instead chose to plead guilty.
"He told police he administered the drug, not she. He has come to terms with the full measure of his criminality."
© AAP 2019