The 20-year-old stabbed his schoolmate at a Choa Chu Kang mall on Tuesday night and waited around coolly for the police to arrive.
His 19-year-old victim was still standing after getting attacked, bleeding from the stomach.
A few minutes earlier that night, four youngsters - three males and one female - had met in front of the McDonald's outlet at Lot One to settle a dispute.
After a heated argument that attracted the attention of passers-by, one of the youths stumbled away, bleeding from his abdomen, reported Chinese daily Lianhe Wanbao.
The victim covered his wound with a white cloth, staining it red.
Mr Frederick See, 32, a customer relations executive in the nearby SK Jewellery store, got to the scene a few minutes after the stabbing.
He told The New Paper that both the attacker and the victim seemed calm.
"The attacker just walked around, waiting for the police to come. I remember thinking, 'he not bad ah, quite brave'," Mr See said.
He said the injured person was standing his ground and stayed on his feet.
He even argued with the female, who is known to be the attacker's girlfriend. At about 9.30pm, the police and an ambulance were called.