18 years ago, Canberra woman Anu Singh killed her boyfriend and watched him die.

In 1997, Anu Singh killed her boyfriend Joe Cinque.

She planned for weeks, told her friends, sedated him and injected him with a lethal dose of heroin. It took 36 hours for him to die.

It has been 18 years and she says she still doesn’t know why she did it.

The couple were living together in Canberra when Singh, a law student at the Australian National University at the time, injected Cinque with the dose of heroin after sedating him with Rohypnol.

He did not die immediately, instead it took more than a day and he died slowly in their bed, while Singh watched on.

“I don’t think you can ever atone for something like that,” she told News.com.au journalist Ginger Gorman when they met in a suburban Sydney coffee shop recently. “What I did was a horrible thing.”

One of the most disturbing aspects of the case was that Sigh’s actions appeared premeditated – she had told a university friend of her plan to kill her boyfriend and later herself.

Word spread among their social circle and on the night of Cinque’s death several of her friends, some of whom knew about the plan, attended a dinner party at the couple’s house.

No one stopped her. No one warned him.

The story is well known, not least because of its bizarre nature – the case was the subject of Helen Garner’s 2004 book Joe Cinque’s Consolation, which will soon be adapted for the big screen.

It is also notable because Singh was not found guilty of murder.

Her charge was instead reduced to manslaughter due to diminished responsibility: she was deemed mentally unfit.

Singh served only four years of a 10-year sentence for her crime and was released in 2001.

Cinque’s parents believe the justice system failed them, in 2011, they spoke to the ABC:

“There’s no rational motivation at all. I was mentally unwell, and I still grapple with that. I still grapple with the whys,” she told Gorman.

“One of the psychiatrists mentioned a state of disassociation, perhaps, like disassociated from reality. I don’t know. There’s no rational explanation.”

Singh suffered from depression. In the months leading up to Cinque’s death, her mother had called a mental health crisis team a number of times.

“I fell through the cracks, as a lot of women do,” she said.

Once incarcerated, she was put on the antidepressant Zoloft and believes she is now “mentally healthy” and “absolutely” takes responsibility for Cinque’s death.

She claims that she was “off the planet mentally unwell.”

“Evil is rare but mental illness is real,” she said. “Having been treated now, adequately I can see how, how really unwell I was.”

She also confided in Gorman that she would like to speak to Cinque’s parents face to face.

“Not [to] seek forgiveness or anything like that, just to be able to say, ‘Look, I’m deeply, deeply sorry for all this’,” she said.

Cinque’s mother Maria wants nothing to do with her.

“She’s a monster. She’s selfish, she doesn’t think about anybody else. Feel sorry? I don’t believe that. She doesn’t,” Maria told Gorman over the phone.

“She’s sorry just because of what happened to her, not because of what she did to my son.”