SINGAPORE - An over-possessive man who could not accept that his girlfriend was seeing other men - despite their agreement to have an open relationship - was sentenced to 13 years' jail on Monday (Nov 12) for killing her during a quarrel.

Neo Chun Zheng, 29, who stabbed Ms Soh Yuan Lin, 23, once in the neck with a folding knife outside his Boon Lay flat on Nov 26, 2015, told the High Court that words could not express his "eternal regret" for what he had done.

"I have completely failed her and I can never forgive myself for all that had happened. It was a toxic relationship but this would not take away my love for her," he said, reading from a note before he was sentenced.

"I miss Yuan Lin dearly and I will always love her with all my heart each and every day."

Neo also apologised to Ms Soh's family. "I am very very sorry for causing you such tremendous grief, sorrow and suffering," he said .

"I can only hope that some day, you may find it in your heart to forgive me."

The Straits Times understands that Ms Soh's family members were not in court.

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It was revealed in court that her family had rejected an earlier apology letter from Neo after he pleaded guilty to a charge of culpable homicide in October last year.

In sentencing, Judicial Commissioner Pang Khang Chau found there was enough evidence of Neo's remorse from his police statements.

The judge imposed 13 years' jail on Neo after comparing the facts of the case to precedent cases submitted by the prosecution and defence.

Neo and Ms Soh, a former air stewardess, met when they started working at the Marina Bay Sands casino as customer relations executives in April 2014.

They began dating but Ms Soh wanted the relationship to be "non-exclusive" and to be kept a secret. Although Neo agreed, he felt she was cheating on him whenever he found out that she was seeing other men.

In March 2015, he circulated compromising photos of her and one of their colleagues after he found out they had been intimate.

In September that year, Neo quarrelled with Ms Soh after he discovered that she was texting someone she had kissed at a club. She broke up with Neo later that month as she felt he was too possessive. She told him she wanted to remain single.

However, Neo found out that she was using dating app Tinder and was angry that she had gone back on her word. He told a friend he thought about disfiguring her with acid.

Neo continued telling Ms Soh how much she had hurt him, and that she had to make it up to him by buying meals for him or going out with him.

On Nov 25, 2015, after she rebuffed his request to watch a movie with him, he texted and called her multiple times until she relented the next evening and bought dinner for him.

Ms Soh went to Neo's flat to deliver a packet of food, but they began quarrelling along the common corridor. Neo's mother, Madam Ng Lian Kee, came out of the flat and told him to let her leave.

But Neo snatched away Ms Soh's mobile phone and hid it inside the flat. He also took a folding knife from his room, hid it behind his back and asked his mother if she knew how many men Ms Soh had slept with.

Madam Ng saw the knife and tried to wrestle it away. But Neo pushed his mother to the floor and stabbed Ms Soh once in the neck. Neo gave himself up when the police arrived. Ms Soh was pronounced dead three hours later in hospital.

After Neo pleaded guilty in October last year, a hearing was held to resolve a dispute over a psychiatric opinion. The court concluded that Neo was not suffering from any mental disorder at the time of the stabbing.

On Monday, Deputy Public Prosecutor Wong Kok Weng sought 15 years' jail, arguing that Neo was exacting revenge on Ms Soh because he felt hurt that he could not possess her exclusively.

But Neo's lawyer, Ms Tan Jun Yin, asked for a term of not more than 10 years, arguing that he was remorseful from the moment he realised what he had done. He tried to stop the bleeding from Ms Soh's neck, called 995 to check on the status of the ambulance and confessed to the police, she said.