SINGAPORE - A mentally ill man who went on a savage rampage on New Year's Eve in 2015, beating his adoptive father to death and assaulting his adoptive mother and his girlfriend, was sentenced to more than 15 years' jail on Thursday (Feb 22).

Su Caizhi, 30, who was adopted by the couple at the age of five and later received $70,000 from their hawker business, has a history of schizophrenia but often did not take his medication.

In sentencing, Justice Kannan Ramesh noted that Su had launched an unprovoked and savage assault against his defenceless 72-year old father, Mr Pang Tee Lin, who had provided for him for more than 20 years.

"The point must be registered with the accused that his recalcitrance in failing to adhere to his medication played a significant part in the demise of the deceased and was a material factor in the length of the custodial sentence," said the High Court judge.

Three days after he was last discharged from the Institute of Mental Health on Dec 9, 2015, Su went off his medication and started experiencing psychotic symptoms and hearing voices.

Despite the warning signs leading up to the tragedy that should have served as "wake-up calls" that he was a danger to those around him, Su did not resume his medication, the judge noted. He added that Su made matters worse by sniffing glue and consuming alcohol on the day of the incident.

A long jail term will ensure that Su is treated in a structured environment and instil in him the need and the discipline to take his medication, to minimise the risk that he poses to others, said the judge.

Su was sentenced to 14 years' jail on a charge of culpable homicide for the killing of Mr Pang.

He was handed 12 months' jail for causing grievous hurt to his adoptive mother Wong Ah Boey, 69, and 15 months' jail for causing grievous hurt to his girlfriend Melissa Foo Fern Yin, 34.

Su was also given six weeks' jail on a charge of wrongful confinement, for a separate incident when he trapped Ms Foo and her former husband inside their flat by putting a padlock on the front gate and pouring glue into the lock on the back gate.

The 12-month jail term will run concurrently with the 14 years' jail. The other jail terms were ordered to run consecutively, making a total of 15 years, three months and six weeks.

Madam Wong was in court with a group of elderly men and women. They left hurriedly after the verdict without talking to Su, and declined to comment when approached by reporters.

Su, who has been jobless since 2014, had pleaded guilty to the four charges earlier this month. Three other charges - for threatening to kill Madam Wong, for spraying black paint on the gates and doors of Ms Foo's flat, and for causing hurt to Ms Foo - were taken into consideration.

Su was adopted by Mr Pang and Madam Wong, who were childless, from China's Hainan province 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 has been admitted to the Institute of Mental Health at least seven times since 2012, with symptoms of hallucinations, delusions, aggressive behaviour and disorganised thought. He spent most of his time on his computer.

Despite this, the couple 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 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.

He then 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 head and banged it against the wall. He also slapped her, punched her face and kneed her in the stomach.

Ms Foo called the police after she regained consciousness and found Mr Pang on the floor. Su was sleeping when police officers arrived.

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

When The Straits Times visited Madam Wong's flat on Thursday evening, the interior was completely dark and the gate padlocked. Nobody appeared to be home.

But neighbours said this was "quite normal" for Madam Wong. Canteen hawker Bob Khan, who had lived there for 34 years, said Mr Pang had always been the more cheerful one.

"Madam Wong kept to herself. Even after the incident, she is still the same."

Additional reporting by Rachel Au-Yong