A woman who killed her ex-partner after drugging him and mutilating his genitals is a "gentle, kind and polite person" who was concerned about the welfare of the children they had together, a court has heard.

Jian Chen, 49, is being sentenced for the manslaughter of 48-year-old Xian Peng in her home in North Ryde in February last year.

Mr Peng was visiting his former de facto spouse after returning from China with his new girlfriend, when she gave him soup spiked with sleeping pills.

Once Mr Peng was asleep, Chen bound his hands and feet and stabbed him a number of times in the neck and groin, before cutting off his penis and scrotum.

The Crown has accepted that at the time of the killing Chen was suffering from "substantial impairment due to an abnormality of mind" but claims she was also acting out of "fear and anger".

But during a Supreme Court hearing to determine Chen's sentence today, her ex-husband, Michael Leung said she was a "loving, caring mother" who had been extremely concerned about the welfare of one of the children she and Mr Peng had together.

Mr Peng had taken the child to China, and Chen was worried that he wasn't being cared for properly, Mr Leung said.

"[The child] was in a miserable condition in China," Mr Leung told the court.

"She found that Peng was spending little time caring for the child. She was very eager to get [the child] back so she could look after him."

When Mr Peng eventually returned to Australia with the child, Chen was extremely anxious that he would take the child back to China and not return, the court heard.

She took the child's passport and locked it away in a safety deposit box.

But the prosecution said Chen was motivated by fear and anger over Mr Peng's alleged sexual exploits.

The Crown prosecutor, Mark Tedeschi, QC, said Chen's abnormality of mind did not remove her culpability for the crime.

He said that, while this illness was the underlying circumstance behind the crime, "there was a strong motive to cause the death of her ex-de facto to prevent the risk of him coming to Australia and removing their son".

"She didn't take any other steps to prevent him from doing that."

He said Chen was also motivated by anger stemming from the mistaken belief that Mr Peng was a sexual predator.

"She was concerned that her husband was a serial predator, believing that he would make financially affluent women pregnant and use the fact ... to extort money from them.

"The atrocious disfigurement which she caused to the deceased's genitals while he was still alive was done in a desire to punish him for his predatory behaviour towards her and other women."