The mother of 22-year-old Christchurch woman, Renee Duckmanton, today told her killer as he was jailed for a minimum of 18 years, "I hope you rot in hell".

Gambian-born butcher Sainey Marong has today been jailed for life for the brutal murder of Duckmanton.

The court heard today that Duckmanton, who worked as a sex worker, suffered from cerebral palsy which made her physically and emotionally vulnerable.

Justice Cameron Mander described Marong's brutal crime as premeditated and predatory.

He noted that Marong had, for some time, thought about killing a prostitute.

His internet searches in the weeks before the murder, which included kidnapping, local prostitutes, how to render a person unconscious, killing with bare hands, past murders of Christchurch sex workers, and necrophilia, seem to show a preoccupation with the killing a sex worker with the ultimate likelihood of fulfilling a "depraved sexual fantasy".

It was a "cold-blooded" and "particularly callous and cruel" murder, Justice Mander said.

Marong's choice of strangulation appeared to match his desire to keep her body intact in order to carry out his depraved fantasy. Setting her body on fire, which caused great pain and distress to her loved ones, shows his complete lack of empathy for his victim.

Marong was sentenced to life imprisonment with a minimum non-parole period of 18 years.

As Marong was led into custody, members of the public gallery reacted angrily to the sentence length.

One man angrily tried to storm the dock while yelling, "Eighteen years is f****** b*******".

A woman yelled, "Rot in there you maggot."

Marong arrived in New Zealand on a fake passport and was about to be deported in May 2016 when he picked up Duckmanton from the city's red light district, killed her and set her body on fire.

Renee Duckmanton killer Sainey Marong was on fake passport, about to be deported

During the harrowing two-week trial at the High Court in Christchurch earlier this year, 33-year-old Marong admitted strangling Duckmanton but denied that he meant to kill her.

He said he had no murderous intent, claiming he was insane at the time, suffering mental impairment and delusional, psychotic thoughts which began after voluntarily stopping his insulin medication early in 2016.

The jury, however, sided with Crown and its "overwhelming" evidence, dismissing Marong's claims that he had been "disconnected from reality" at the time he killed 
Duckmanton, and that from May 6-17, he was poisoned by kidney failure that resulted in urine "travelling in my brain".

Insanity plea ruled out in Duckmanton trial

Today, as Marong was sentenced, the packed courtroom heard emotional victim impact statements from Duckmanton's devastated family.

Her "very proud" mother Tracy Duckmanton misses her "beautiful, goofy, bubbly, loving, sweet, kind, thoughtful, naïve" daughter every day.

She directed her emotional words to Sainey in the dock.

"Look at me, Sainey," she said.

"You listen to this. You came into her life and your actions devastated my life and my family's life. Nothing will ever be the same.

"Nothing, no sentence, no punishment will ever be enough.

"I hope you rot in hell."

Her grandmother, Patricia Duckmanton spoke on behalf of the family to say that Marong's actions were "unforgivable".

"No sentence in our minds will be enough because you have given us and Renee a life sentence of loss," said Duckmanton's 'Nana Pat'.

Her father Brent McGrath said he will be "forever haunted by the way she died".

He addressed Marong as a "maggot" and told him his lack of remorse is "more than evident".

A psychiatrist and a psychologist, who both assessed Marong before he stood trial, concluded that he did not meet the criteria for a defence of insanity.

Forensic clinical psychologist Ghazi Metoui found that Marong reported a settled mental state over the period "leading up to, during and subsequent" to the killing.

Dr Erik Monasterio, a forensic psychiatrist, found Marong's claims of insanity to be "farcical" and "preposterous".

His DNA matched samples taken from Duckmanton, and was also directly linked to samples found where her body was dumped.

Marong, whose internet history in the days before the killing showed searches for necrophilia, denied having sex with her dead body.

His movements on May 14, 2016 were recorded on CCTV as well as cellphone and bank records.

During his evidence, Marong said that after he had sex with Duckmanton in the back seat of his car, she wanted to return to Manchester St.

He said "that voice, that screaming, that yelling was agitating me" and the dispute "triggered something very serious".

"The only method I could use to stop it was to compress her neck," he said.

Marong arrived at Auckland Airport on a flight from Melbourne via Hong Kong on a fake passport on January 10, 2014.

He destroyed the passport at the airport, the court heard.

While Marong had been living and working in New Zealand legally for a period, at the time of Duckmanton's murder he had been in the country illegally. He had been issued with a deportation notice, it's been confirmed.

Marong was born and raised in Gambia's capital city of Benjul as a member of the Mandinka tribe.

After secondary school, he studied computer science and commerce at university before dropping out to drive a taxi.

He left Gambia on August 28, 2012, leaving behind a wife and three children.
After going to neighbouring Senegal, he went to Hong Kong for 14 months before landing in New Zealand.