SINGAPORE - A 69-year-old retired aircraft technician who killed his wife of 36 years while he was suffering from psychotic delusions was on Monday (Oct 16) given two years' jail, one of the lightest sentences meted out for culpable homicide.

As the jail term was backdated to the date of his remand on March 13 last year, Kong Peng Yee could be released from prison as early as on Monday (Oct 16), with one-third remission for good behaviour.

"For them, it's an unexpected surprise," defence counsel Sunil Sudheesan said of the reaction from Kong's family members. They declined comment when approached by reporters.

Kong pleaded guilty last month to culpable homicide by slashing Madam Wong Chik Yeok's head, first using a knife and then a chopper, at their Sengkang home on March 13 last year.

Prosecutors had sought at least nine years in jail, while the defence asked for less than five years.

"I think that punishment is probably not the most appropriate response to a man like the accused here, and certainly not the nine years' imprisonment sought," said High Court judge Choo Han Teck.

The court had earlier heard that Kong developed delusional beliefs that his family was trying to harm him and decided to make a pre-emptive strike by killing his 63-year-old wife.

Kong told police that his mind told him to "make sure she die" and that he was "happy" on realising that she was dead.

He was initially charged with murder but the charge was reduced, as a court-mandated psychiatric assessment by Dr Kenneth Koh from the Institute of Mental Health found that Kong's mental responsibility for his actions had been substantially impaired by his psychotic delusions.

Kong's psychiatric disorder is now in remission with medication and he has been certified safe to be returned to the care of his family.

On Monday, Justice Choo waxed philosophical as he delivered his decision on the appropriate sentence to be handed down to Kong.

"What is the appropriate punishment for a man whose act was guided by thoughts that entered unbidden into his mind? There is no clear answer," he said.

Justice Choo noted that Kong was suffering from a psychosis that affected his mental responsibility for the killing. "Given those circumstances, a reasonable man may fairly wonder why any punishment is even required? His madness is its own punishment."

The problem, said the judge, lies with "an archaic law that has been incorporated into our statute".

He was referring to a legal test, known as the M'Naghten rule, laid down in a 19th century English case which declared a man not to be insane if he either knew what he was doing or that what he was doing was wrong.

"From that moment on, legal insanity and medical insanity have not fitted themselves snugly in the same box," he said.

The judge added that there are many kinds of illnesses and conditions that can create an abnormality of mind, and not all abnormalities affect the sufferer's mental responsibility to the same extent.

Justice Choo said the language of the M'Naghten rule should be re-examined.

"Doctors and lawyers should speak a common language when dealing with the mental responsibility of an accused who was labouring under a mental illness at the time of the offence," he said.

In Kong's case, after undergoing cataract surgery in January last year, he refused to take medication for other ailments, among other things, believing that laxatives meant for constipation were poison.

Two months later, he collected his health check results and continued to worry that someone was trying to harm him or that he was going to die from a disease.

While at his older daughter's home, he told her that he did not think she was his biological daughter. The next day, Kong started behaving strangely while at church.

After he returned home, he took a nap but woke up to roaring sounds.

He took a knife from the kitchen sink and stabbed Madam Wong until she was dead. An autopsy noted 189 injuries, including knife wounds and bruises. She bled to death from multiple slash wounds to the head.

Kong told police he believed that he should kill his wife first because his family might want to kill him.