He set fire to his family home in 2009 and his father died in the blaze.
Ho Wei Yi, 33, who has a history of mental illness, pleaded guilty of culpable homicide not amounting to murder in the High Court yesterday.
In an unusual twist, his brother and wife wanted the court to give him a heavy sentence.
During the sentencing hearing on Thursday, Deputy Public Prosecutor (DPP) Tan Wen Hsien submitted an appeal written by Whampoa MP Heng Chee How on behalf of Mr Jeremy Ho Wei Chun and his wife Serene Tiong Sze Yin.
In the letter, dated Jan 20, Mr Heng said the couple had told him that Ho and his mother, who is living with the brother's family, are mentally unwell.
Mr Ho and Madam Yin had learnt that Ho, who has been in prison for the last five years, would be sentenced soon.
They had also learnt that their mother had engaged a lawyer to represent Ho to appeal for a lighter sentence.
"They hope that the brother will not get a lighter sentence as they are not able to support two mentally unwell persons. They also have a young son to look after," Mr Heng wrote.
"They said that the brother might pose a danger to the family and they are very worried." The letter drew strong objections from Ho's lawyer, Mr Josephus Tan.
In an emotionally charged rebuttal, he argued that the brother and his wife did not live with the family at their flat on McNair Road in Boon Keng.
He urged the court to impose a jail term of four to six years for Ho and said that Ho's mother and a cousin would be able to care for him if he is given an early release.
"No need to be emotive," Justice Tay Yong Kwang said. "We'll take it that the brother is not going to take care of him."
He then sentenced Ho to eight years in jail.
The sentence takes effect from September 2011, the day he was released from the Institute of Mental Health (IMH).
There will also be no caning, Justice Tay said.
Ho could have been sentenced to life imprisonment or up to 20 years in jail, and caning or a fine.
Throughout the exchange, the fair-skinned, bespectacled Ho did not show any emotion, even when details of the blaze which killed his 58-year-old father, Mr Michael Ho Shiong Chun, were read out.
In a report dated Aug 31, 2009, Dr Todd Tomita of the IMH had diagnosed Ho as mentally ill and suffering from schizophrenia.
Ho was first admitted to IMH in 2003.
On the night of Aug 5, 2009, Ho said he heard voices coming from the bed in the master bedroom of the family flat.
Wanting to rid the flat of the "evil spirits", the jobless man brought two pieces of paper into the room, placed them on the bed and set them on fire with a lighter.
The voices in his head got louder and he fled the flat. Before leaving, he padlocked the front gate, trapping his father.
The court was also told about incidents of violence that Ho had committed against his family. His condition, the prosecution said, deteriorated in 2006 after he stopped taking his medication.
Resenting his parents for making him undergo electroconvulsive therapy (ECT), Ho resorted to "increasingly hostile and violent behaviour".
ECT is a treatment of mental illness by the application of electric shocks to the brain.
In the early hours of July 2, 2009, after an argument about his ECT treatment, Ho attempted to force his father to swallow one of his old psychiatric pills. He then hit his mother on the face and tried to swing a chair at her.
Later that day, his parents went to the Family Court to apply for a Personal Protection Order against him. Another dispute occurred on Aug 5 - the day of the fire - at around 6pm, the court heard.
Afraid that Ho would turn violent again, his mother left the home. When she returned at around 8.15pm, her husband told her that Ho had gone out.
It was the last time she saw her husband alive. She went out again and was informed at 11pm that the flat had caught fire.
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