SINGAPORE — Although they were in an open relationship, he was unhappy the former air stewardess was dating other men. And even though they had broken up, a heated quarrel over this issue led Neo Chun Zheng, 28, to plunge a knife with a 9cm-long blade into Ms Soh Yuan Lin's neck.

How the 23-year-old was killed along a common corridor outside Neo's flat two years ago came to light for the first time on Tuesday (Oct 17), as he pleaded guilty to a reduced charge of culpable homicide.

Originally charged with murder, Neo told the court on what was to have been his first day of trial last week that he would plead guilty to culpable homicide. The maximum sentence for the latter is life imprisonment and caning, or up to 20 years' jail, caning or a fine. He could have been hanged, if found guilty of murder.

The couple got into an open relationship in 2014, after meeting on the job at Marina Bay Sands Casino, where they had worked as customer relations executives in the player development department. A few months later, arguments became frequent, and by the end of September 2015, Ms Soh broke up with Neo.

She found Neo was "too possessive and overly-controlling in their non-exclusive relationship", said Deputy Public Prosecutor Dwayne Lum.

Around November, Neo told friends at work that he wanted to get even with Ms Soh, for hurting him by dating other guys while she was with him, and subsequently breaking up with him.

He had also, on multiple occasions, told his friends that he wanted Ms Soh to "die an ugly death" in Mandarin, the court heard.

During that same period of time, Neo also repeatedly told Ms Soh how much she had hurt him and that he wanted her to make it up to him by returning favours he had done for her in the past.

Ms Soh gave in to the requests and agreed to perform favours for Neo, such as going out with him or buying meals for him.

On Nov 26, Neo peppered Ms Soh with numerous calls and text messages in the day. That night, she bought dinner for him. And just past 8pm that day, Neo and Ms Soh got into an argument over their relationship problems along the common corridor outside his Boon Lay Drive flat.

Neo blocked her way to prevent her from leaving. When she tried to use her mobile phone to text her mother, Neo snatched the phone from her and hid it in the house before returning with a knife from his room.

Neo's mother quickly stood between the pair and tried to wrest the knife from her Son. In the ensuing struggle, she knocked Neo's glasses off and was shoved to the ground in retaliation.

Then, Neo swung the knife at Ms Soh, plunging it into her neck. She collapsed to the floor face-first and started bleeding from her neck and mouth.

Neo threw the knife down the rubbish chute and used a towel to apply pressure on his ex-girlfriend's neck to stop the bleeding.

When police officers came, Neo surrendered himself and confessed that he had stabbed Ms Soh.

She died in hospital three hours later. Autopsy reports showed a stab wound measuring 3.4cm-wide on her neck, which completely cut her jugular vein.

In court on Tuesday, the bespectacled Neo spoke softly into the court microphone: "I intend to plead guilty." And as he was convicted of culpable homicide, Neo showed no emotions, and kept his head hung low.

He was allowed 20 minutes after the hearing ended to speak with his family. He was later seen talking to his father, while another family member looked on.

Neo will be sentenced at a later date because his lawyers are disputing a psychiatric report from 2015 finding Neo had no mental disorder at the time of the offence.