SINGAPORE - A jobless man with a history of schizophrenia and alcohol abuse went on a violent rampage on New Year's Eve 2015, beating his adoptive father to death and assaulting his adoptive mother and his girlfriend.

Su Caizhi, 30, was taken in by the couple at the age of five and later received $70,000 from their family business, but "repaid their kindness with savagery", a court heard.

He pleaded guilty on Monday (Feb 12) to four charges, including culpable homicide for killing his father Pang Tee Lin, 72, causing grievous hurt to his mother Wong Ah Boey, 69, and causing grievous hurt to his girlfriend Melissa Foo Fern Yin, 34.

The High Court heard that Mr Pang and Madam Wong, who were childless, adopted Su from Hainan province, China, in 1992.

They changed his name to "Pang Kee Hiang" but he changed it back in September 2015, a few months before the tragedy.

Su, who has been admitted to the Institute of Mental Health at least seven times since 2012, has been unemployed since 2014 and spent most of his time in his bedroom on his computer.

He showed symptoms of hallucinations, delusions, aggressive behaviour, and disorganised thought, but often did not take his prescribed medication. He also had a history of alcohol abuse.

Despite this, the couple, who were hawkers, gave him $70,000 out of the $100,000 they received from selling their curry rice stall when they retired in August 2014.

He lost part of the money in forex trading and two failed businesses, paid for an accounting course which he did not complete, and spent the money on himself and Ms Foo, with whom he had an on-off relationship.

On the morning of New Year's Eve, 2015, when Ms Foo went over to the family's Bedok Reservoir flat, Su was angered when she refused to engage in a particular sex act with him. He began assaulting her and she passed out when he grabbed her neck, but he continued to punch and kick her - and even stepped on her face.

After attacking Ms Foo, he turned his anger on his father after voices in his head told him that Mr Pang had hidden his medicine.

Su stormed into the master bedroom, shouting at his father for taking away his medication and punched him in the face. When the elderly man fell to the floor, Su stomped on his face.

As Mr Pang lay bleeding from his head, mouth and nose, Su kicked him in the chest before returning to his room.

A shocked Madam Wong went to Su's room to ask what had happened but Su grabbed her head and banged it against the wall. He slapped her, rained punches down on her face and kneed her in the stomach, telling her in Mandarin: "I want to hit you till you die".

He then returned to his room and lay on his bed while his mother remained dazed on the couch.

Ms Foo, who regained consciousness and fled from Su's room while he was attacking Madam Wong, called the police after finding Mr Pang motionless on the floor.

When police arrived, Su was sleeping in his room. He was uncooperative when the officers called out to him, telling them that he did not want to be disturbed.

The officers arrested him for causing grievous hurt and found four packets of glue and two empty beer cans in his room.

Mr Pang was warded in the surgical intensive care units at Changi General Hospital, with severe brain injury and fractures to his face, ribs and spine. Due to his poor prognosis, his family members agreed to take him off life support nine days later.

On Monday, Deputy Public Prosecutor Sarah Ong urged the court to impose at least 18 years' jail. She said that after years of toiling, the elderly couple suffered tragic fates at the hands of a man they had taken in as a boy and raised. "He repaid their kindness with savagery", said the DPP.

She argued that a lengthy jail term is needed to provide Su with a structured environment to ensure that he complies with his treatment, as well as to protect the public.

Su's assigned lawyer, Mr Nakoorsha A.K., sought a sentence of not more than 12 years, arguing that his client's actions were not premeditated.

He said Su was traumatised and felt abandoned when, as a child, his birth father left him in Singapore. Mr Nakoorsha said Su had an "emotionally distant" relationship with his adoptive parents. He added that Su hopes that his biological parents will still be alive when he is able to visit them after being released from prison.

A date has yet to be fixed for his sentencing.