A mentally ill cleaner, who stabbed his co-worker after hallucinating that she was taunting him to kill her, pleaded guilty to culpable homicide on Thursday for the 2011 killing at Jurong East MRT station.

Pua Kok Heng, 57, became angry after Madam Goh Ah Moy, 62, who worked at the MRT station, scolded him for using her cleaning equipment.

Pua, who was suffering from paranoid schizophrenia and believed that he is a victim of political persecution, confronted Madam Goh with a knife after seeing a "floating" image of her talking to him.

His first confrontation, on Nov 23, 2011, was interrupted by their supervisor who told him he was fired and then led him away.

The next day, Pua again saw an image of Madam Goh "flying in" from the kitchen window of his rental flat and taunting him.

This time, armed with two knives, he approached Madam Goh, who was resting during her lunch break, and shouted her name. He stabbed her once in the chest and pulled out the knife. He was arrested that evening at his rental apartment.

A government psychiatrist said that Pua's persecutory delusions were "significantly causally linked" to his offence.

Deputy Public Prosecutor Jasmine Chin-Sabado on Thursday sought a sentence of life imprisonment, arguing that Pua, who requires long term psychiatric treatment, poses a high risk of future violence to the community.

Pua's lawyer Josephus Tan said his client had been taking his medication and his condition has improved significantly.

Sentencing was adjourned to a later date.