Weijie He stabbed 19-year-old student Liqun Pan in the southern Sydney suburb of Wolli Creek on June 27, 2020, before he fell from the fourth floor of the apartment building.
An emotional statement from Ms Pan's father was read out in the NSW Supreme Court on Tuesday, as her killer listened on wearing prison greens and seated in a wheelchair.
"Our hope and happiness evaporated the moment her life was viciously taken away," Zewu Pan said in the statement, which was written in Mandarin but read to the court in English.
"The news of her death brought devastation to our entire family.
"The immeasurable pain crushed everyone. We couldn't accept it was true."
Coming from a family of farmers in Guangdong province, Ms Pan moved to Australia to study English.
Her family was supportive of Ms Pan's desire to get a better education, the court heard.
However, the kind, intelligent and conscientious woman's life was cut short with her brutal murder.
"Why is God so unfair to her?" her father said.
"Why is God so cruel to us?"
As for the "vicious and heartless murderer", Mr Pan said he hoped to see him rot in jail for the rest of his life.
"He must be made to pay for his evil deed."
The court heard He was a regular user of nitrous oxide, commonly known as laughing gas, consuming five boxes two days before the murder.
Defence barrister Geoff Harrison argued his client had experienced drug-induced psychosis at the time of the killing.
But forensic psychiatrist Adam Martin told the court the Chinese national had shown "a degree of control and coercive behaviour" towards Ms Pan before the murder.
He argued with his girlfriend, believing she had a "sugar daddy" and threatened to beat her up if she did not complete her chores, according to agreed facts filed with the court.
On one occasion when she flew back to China, Ms Pan signed a contract that she would not go to bars, drink alcohol or do anything with people of the opposite sex.
"Is it possible, doctor, that the murder had nothing to do with nitrous oxide?" asked crown prosecutor Rossi Kotsis.
"Yes," Dr Martin replied.
Justice Julia Lonergan will have to consider whether He's moral culpability was reduced because of the claimed psychotic episode, or if he was merely intoxicated by the laughing gas.
While He pleaded guilty to murder, the killer had also been coached by his family to make denials and claim he had amnesia.
In December 2021, He told Dr Martin that both he and Ms Pan had been harmed by "bad people", who killed his girlfriend and threw him off the building.
The hearing continues.
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