For seven months, they shared a living space, sleeping just metres apart from each other in an eldercare home.
When his roommate turned off the light while Sim Hwa was making himself a hot beverage, the 89-year-old saw red.
Sim simmered with anger for two hours before attacking Mr Lee Say Fong, also 89.
First, Sim threw hot water on him, then he slashed him with a fruit knife and a chopper.
The attack left Mr Lee with 26 per cent second-degree burns over his chest, back, right shoulder, right arm and face. He also had cuts on his chest, right ankle and left leg.
Sim, who had been staying at Kheng Chiu Happy Lodge at Block 70, Tampines Avenue 4 since 2011, was sentenced to 18 months' jail yesterday. He pleaded guilty to an amended charge of causing hurt to Mr Lee.
Deputy Public Prosecutor Li Yihong said that, at about 9.30pm on Nov 23 last year, Sim was making himself a mug of Milo in the living room of the fifth-storey flat when he became angry with Mr Lee for turning off the lights.
Two hours later, Sim took a kettle of hot water to the bedroom and splashed the contents on his roommate, who was seated on his bed.
Sim then picked up Mr Lee's 14.5cm-long fruit knife and slashed him a few times across his chest and left hand.
After that, he used a 16cm-long chopper to slash him across his legs a number of times.
Sim left the room and disposed of the two weapons in a laundry basket outside their room. He went to the ground floor and informed the staff that he had hit someone.
Meanwhile, an injured Mr Lee made his way up to the sixth floor and knocked on the bedroom door of a staff member, who called for an ambulance after seeing that he was bleeding profusely.
Mr Lee suffered extensive burns and underwent multiple operations. He was admitted to the Burns Intensive Care Unit at Singapore General Hospital.
He was discharged on Jan 10 for step-down care at St Andrew's Community Hospital after being warded for close to 50 days. Mr Lee returned to the home only in late February.
Pleading for leniency, Sim's lawyer, Mr Liaw Jin Poh, said in his mitigation plea that his client was stricken with multiple illnesses.
He said Sim had lived in constant fear of a roommate who had previously assaulted and injured him, and was a source of constant provocation and bullying. Sim also said that Mr Lee kept knives in the room.
Mr Liaw said the father of five had been let down by his family, who kicked him out of his house.
"Although his five children are all educated and gainfully employed, the accused had to make an application to the Maintenance of Parents Tribunal before the children agreed to give $50 each for his subsistence and medical bills, etc," said the lawyer.
Sim's sentence was backdated to Nov 25. He could have been jailed for up to seven years and fined.
Elderly people serving jail time may be located in the prison's medical wing for safety reasons. The food served to them may also be slightly different to suit their nutritional needs.