A jobless man with a history of schizophrenia and alcohol abuse went on a violent New Year's Eve rampage in 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" after hearing voices in his head, a court heard.

He pleaded guilty to four charges yesterday, including culpable homicide for killing his father Pang Tee Lin, 72, causing grievous hurt to his mother, Madam Wong Ah Boey, 69, and causing grievous hurt to his girlfriend, Ms 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. He spent most of his time 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 some on himself and Ms Foo.

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 sex act with him.

He began assaulting her and she passed out, but he continued to punch and kick her.

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 and punched his father in the face. When the elderly man fell to the floor, Su stomped on his face and kicked him in the chest before returning to his room.

Madam Wong went to Su's room to ask what had happened but he grabbed her by her head and banged it against the wall.

He slapped her, rained punches on her face and kneed her in the stomach.

Ms Foo, who had regained consciousness, called the police after finding Mr Pang on the floor.

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

Mr Pang suffered severe brain injury and fractures to his face, ribs and spine. He was taken off life support nine days later.

Deputy Public Prosecutor Sarah Ong urged the court to impose at least 18 years' jail.

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

Su will be sentenced at a later date.