He saw an image of his colleague flying in from his kitchen window and telling him that she wanted him to die.
Pua Kok Heng, 57, who had just been fired from his cleaner job the day before, got angry.
He had been dismissed after threatening his colleague, Madam Goh Ah Moy, 62, with a knife.
Pua then tucked two knives into his waistband and headed to Jurong East MRT Station to look for Madam Goh.
When he found her, he stabbed her in the chest.
She died at the National University Hospital about an hour later.
Pua, who suffers from paranoid schizophrenia, yesterday pleaded guilty to culpable homicide on Nov 24, 2011.
Two days before the fatal attack, when he was still working as a cleaner at the MRT station, Pua used Madam Goh's equipment to clean the plant rooms.
She later scolded him for dirtying her mops and pails, and they had a heated argument.
Pua's colleague, Mr Rodney Tan Kim Khuay, 70, intervened before the matter got out of hand.
Mr Tan was also Pua's flatmate.
At home the next day, on Nov 23, 2011, Pua had a hallucination of Madam Goh floating into his rental flat in Marsiling.
The apparition "challenged" him to go to Jurong East MRT Station with a knife.
Pua took a knife and went to the MRT station.
At 8.30am, his supervisor, Mr Simon Lim Kim Heng, saw Pua shouting and hurling vulgarities at Madam Goh in the public concourse area.
She appeared stunned and was rooted to the ground.
When Mr Lim asked Pua to calm down, he told his supervisor not to interfere and took out the knife.