A maid, who almost got away with killing her employer's mother-in-law, whom she smothered with a pillow while she was sleeping, was yesterday sentenced to 10 years' jail.

Indonesian Yati, 24, who goes by one name, pleaded guilty in the High Court to culpable homicide not amounting to murder for killing 76-year-old Aandi Abdul Rahman Rasheeda Begam between midnight and 2am on Jan 14 last year . The crime came to light only because Madam Aandi's daughter-in-law kept questioning Yati about it.

After her arrest, Yati had various versions about why she did it, telling a psychiatrist she had heard voices telling her to kill Madam Aandi, or that she had killed the elderly woman so she would be treated better. She said her employer, Madam Mohietheen Fathima Kuny Maidin, disliked her mother-in-law, so by killing her, she would "win ma'am's heart".

Senior Judge Kan Ting Chiu noted that "by deliberate action and intention and sustained effort", Yati took the life of an aged victim "for no reason that we can really understand".

Yati, married with a daughter, came to Singapore in 2013 when she was 22. Her first job lasted five months; her employer sought a replacement as Yati was "odd".

In October 2013, Yati started working for Madam Fathima's family at their five-room Serangoon North flat. She shared a room with Madam Aandi, who used a wheelchair and had medical conditions such as heart disease.

Yati said she felt stressed as she was scolded for her mistakes and was given leftover food.

On the morning of Jan 13 the following year, she decided to kill Madam Aandi. That night, when the elderly woman was asleep, Yati grabbed the pillow she was sleeping on and smothered her with it. After checking that she was dead, Yati went to the living room, greeted her employer, and went back to sleep, waking up at 5am to do her daily chores. When Madam Fathima could not rouse her mother-in-law, Yati feigned ignorance.

A doctor certified the cause of death as heart disease as he could not find any visible injuries. Madam Aandi was buried that afternoon at a Muslim cemetery. But Madam Fathima continued to press Yati about what had happened, and she finally confessed on Jan 16.

An autopsy on Madam Aandi's body, exhumed on Jan 18, could not determine how she died. The forensic pathologist said smothering often leaves no signs.

Yati was found to be suffering from major depressive disorder, which impaired her mental responsibility for her acts.

Dr Stephen Phang from the Institute of Mental Health said she was likely mentally unwell before she came to Singapore. He recommended at least two years of robust psychiatric treatment. "In short, she is a dangerous individual, especially if left untreated."

Yati's lawyer, Mr Mohamed Muzammil Mohamed, said she was overworked and underfed.

But Deputy Public Prosecutor Chee Min Ping said Madam Fathima refuted Yati's allegations of ill-treatment.