An 82-year-old mentally ill retiree who stabbed his 54-year-old daughter-in-law to death, following a quarrel over the washing of curtains, was yesterday jailed eight years.

Char Chin Fah pleaded guilty to culpable homicide for killing Madam Ong Guat Leng, a housewife who was married to his elder son Yeng Kong, 52, on Aug 21, 2014. He was found to have suffered an acute paranoid reaction, which reduced his mental responsibility for his actions.

In sentencing, Justice Woo Bih Li said the very serious offence would ordinarily attract a long jail term but he "experienced some hesitation" due to Char's advanced age.

The High Court heard that Char lived with his elder son's family in Tampines for two years, and had an acrimonious relationship with his son and daughter-in-law.

Char's personality made it easy for him to develop paranoid and abnormal suspicions about the intentions of others. This made him "a difficult subject to live with", said a psychiatric report.

He often complained to his two other children about Madam Ong, whom he felt was disrespectful as she called him by his full name and often nagged at him.

Char behaved in socially inappropriate ways, such as placing his dirty socks on the dining table and not closing the door when using the toilet. This annoyed Madam Ong, who was on medication for an anxiety disorder, which made her nag whenever she felt agitated.

Char was given a bed in the flat's living room and his living space was separated from the rest of the living room by a curtain.

The tension reached boiling point on the evening of Aug 20, 2014, when the couple argued with Char for not washing the curtains.

Feeling betrayed by his son for not speaking up for him, Char decided to kill Madam Ong. At about 5.30am the next day, Char left the flat for his daily exercise. Over breakfast, he wrote a "suicide note" to his daughter and drank beer to "gather more courage".

Char went back to the flat when Madam Ong was sleeping alone at home. He took the sharpest knife in the kitchen, went to the master bedroom and attacked her.

He grabbed her by the hair and repeatedly banged her head against the wall. She ran to another room but he caught up with her. She begged him for forgiveness, but he did not relent.

Madam Ong ran back to her room but was unable to close the door and Char stabbed her in the chest with the knife, which had a 20.4 cm blade. He left the knife in her body, washed up and called his daughter and younger son to confess.

The daughter, Madam Jenny Char, called Madam Ong's daughter, who called the police. Meanwhile, Char went to his daughter's home to hand her his belongings.

An autopsy on Madam Ong found four stab wounds in her chest, fractures on her skull, as well as bruises, abrasions and swellings on her face and body.

Dr Stephen Phang of the Institute of Mental Health said that though his "perceived villainess has been eliminated", it was possible for Char to develop paranoid reactions to family members in the future.

Prosecutors said 10 years' jail was fair but defence lawyer Ramesh Tiwary sought five. Char's daughter will house him after he is released, he said. Char's daughter and younger son were in court yesterday but not his elder son.

Madam Char, 53, said they do not keep in contact with their brother. She said her father - who worked as a tailor, bus driver and coffee shop attendant - had a "temper" and wanted to be respected, but denied that he was difficult to live with.