It was the sheep's tongue beside the smouldering corpse that baffled most. Flaccid, grotesque bluish gristle, it seemed so out of place yet somehow so inexplicably linked.

Detectives noticed it two metres from Renee Duckmanton's singed body. They placed a tiny cone beside it, noted a "piece of flesh", "meat", and moved on with their painstaking scene examinations, all the time wondering, What the f*** is that all about? A discarded coincidence, perhaps? Or a killer's deliberately planted calling card, a sickening signature?

All murders are hard to explain. There is surely an argument of insanity at the point anyone takes another human being's life. But there's usually a reason, a motive: jealousy; greed; white rage; revenge; power; self-defence.

A slide into insanity was what Gambian-born butcher Sainey Marong tried for, in retrospect at least. Desperately trying to stack a story together of building instability in the months and days before he picked up sex worker Duckmanton from Christchurch's red-light district and strangled her to death, dumped her body on a country roadside, and set her alight.

The tongue belonged to a sheep that Marong had paid to be slaughtered the day before. After the Halal butcher had chopped it up, Marong asked to remove its tongue himself, with his own knife, elucidating that it had just looked so … "marvellous".

Stopping insulin medication had caused insomnia and lack of appetite which kept the diabetic awake and caused him negative thoughts, cruising for girls all night, he tried.

The jury stared blankly.

Kidney failure poisoned his body and sent urine travelling round his brain, he tried.

The jury stared blankly. One jotted a note.

A photo of Renee Duckmanton taken one week before her death. (Photo / Supplied)

"Mr Marong indeed was way off track," Marong repeatedly said, in third-person narration, variously explaining away his killing with phrases like, "I was out of my sense", "living in a whole new world", "disconnected from reality", "my partial conscious mind was swaying my delusional frame", "serially agitated", "symptoms of impulsivity", "flight of ideas". He cited an array of medical conditions and terminology too, straight from a textbook: psychomotor agitation, psychosis, delusions, hypoglycaemia, delirium, paranoia, mental impairment, urine toxicity, hypersexuality.

The 33-year-old became increasingly agitated when it appeared nobody in the courtroom understood his internet history: hundreds of searches for local escort girls, prostitutes, massage parlours, necrophilia, chloroform, how to kill someone with bare hands, and articles on other Christchurch sex worker murders, and one entitled 'How to kidnap a girl: an informative guide'.

Everything he did, Marong said, was beyond his control. He was insane. Just at that time, however, around the first five months of 2016. He is fine now though, he said.

Forensic psychiatrist Dr Erik Monasterio called his insanity claims "farcical". The expert added that if Marong were to be believed, he would be a walking "encyclopedia of mental disorders".

Ghazi Metoui, a forensic clinical psychologist who spent hours talking to Marong, found him to be an ego-centric narcissist with a considerable sense of entitlement and grandiosity. He was particularly struck by Marong's "complete lack of empathy", and the "callousness and ease in which he cast aside people from his life". Marong allegedly told him that the prostitutes he was obsessed with – hiring their services many times a week - were "dirty, immoral and inferior" to him.

The first question Crown prosecutor Pip Currie asked Marong on the witness stand – podgy, in yet-another crisp white shirt and tufted black hair - was whether he would agree with an assessment that he was "above average intelligence".

Aware of the glaring eyes – judge, jury, lawyers, journalists, police, victim's furious family – he smiled and tried to play it down: "That is not how I would describe myself but I am aware of so many things in life."

But not, it seems, the price of a human life.

Born in Gambia, a small West African slither on the Atlantic coast, surrounded by neighbouring Senegal, much of Marong's life and how he came to New Zealand in 2012 cannot be reported because of suppression orders.

A Muslim father-of-three, former taxi driver and halal slaughterman, his butcher friends told of a generally happy colleague, quick to share a laugh. He liked lifting weights and cruising in his silver Audi, spending time both in Canterbury and Auckland.

But around January 2016, while living in a shared flat in the suburb of Ilam, it all started going wrong for Marong. At least, that's when he says his descent into madness began.

Security camera footage of Renee Duckmanton walking on Peterborough St in Christchurch at about 9pm on May 14, 2016. (Photo / NZ Police)

After admitting killing Duckmanton, strangling her to death, he was asked if he blamed others for his downfall. He shrugged in the witness stand and said solemnly, "Everything is sad that happened to Mr Marong."

Just as Duckmanton's family and friends wriggled with fury, fists balled, in their public gallery seats that this murderer, the man who'd taken away their precious daughter, sister, cousin, aunty, granddaughter and friend was again feeling sorry for himself, he added, "And to the victim as well."

In May 2016, Renee Larissa Duckmanton was living with her boyfriend Samuel Doak and some friends at a flat in the Cashmere area of Christchurch, at the foot of the picturesque Port Hills. It was a leafy property, surrounded by trees and in the early mornings, often when she returned from working late on Manchester St, alive with birdsong.

Doak says she'd been working as a prostitute for about a year. Her earnings funded their drug habits: methamphetamines, synthetics and chemicals. It also paid for rent, food and cigarettes.

Using assuming a working name of Alena or Alana, the 22-year-old would usually go out twice a week – Thursday and Friday nights. Duckmanton was watched over by a white-haired man, Terry McGowan. He'd stopped to talk to her "on a whim" one night on his way home from the cinema. They chatted and got on well.

"She was an amazing person, and um, we swapped phone numbers," McGowan said.

Duckmanton's father, Brent McGrath, agreed, describing her as "adorable". Her cousin Tracey Lee McGrath said Duckmanton was "really bubbly, really happy, a lovely girl", and although 22, and streetwise, still "pretty much a kid herself".

If her minder McGowan wasn't busy, he would pick her up in the evenings and take Duckmanton to her usual spot at the intersection of Manchester and Peterborough streets. She would leave her handbag with McGowan and text him – and boyfriend Doak back at home – job details. It was her safety net.

McGowan picked her up at about 8.30pm on May 14, 2016. She'd just woken up and was wearing a thin leather black hoody with fur trim, a white singlet, shorts, black shoes, and two necklaces. She'd just dyed her hair a dark red. She looked pretty good, Doak said.

"I gave her a hug and a kiss at the door and that was it," he said. He never saw her again.

A few weeks earlier, a client had locked his car doors on her and she'd had to walk home. But when the same customer, Marong, showed up at 9pm on May 14, 2016 – after cruising past several times - she had no hesitation in taking the job: $100 for sex. She texted Doak.

Eight minutes later, however, she phoned her boyfriend to say the John had changed his mind, and wanted to drive to his house. The job was now worth $300. She also texted McGowan.

Doak says they'd agreed she wouldn't do home jobs. "But that night, we didn't follow that," Doak said.

Marong stopped at an ATM on Lincoln Rd and withdrew $300. The transaction, like most of the murderer's movements that night, was captured on CCTV and in bank records.

A worried Doak phoned her back. "Yeah, yeah I'm fine," she told him. "Stop ringing because he's getting annoyed."

Her voice was "strongly agitating" Marong. He wanted to visit a friend at Rolleston, south of Christchurch, but said they would stop and have sex. Around the Templeton area, Marong says, he pulled over to the side of the road and they had sex in the back seat of his car.

Afterwards, Marong claims that as he went to climb forward into the driver's seat, Duckmanton grabbed his shirt, and told him she wanted to return to Manchester St, saying the "job is done".

"I insisted that I would have to go where I agree on. She started yelling at me," Marong claimed. "I was in resistance, impulsive. That voice, that screaming, that yelling was agitating me. The only method I could use to stop it was to compress her neck."

Marong says he snapped. "I have no motive to cause any harm, I just did that to shut her [up]."

The Crown claims it was a fulfilment of a twisted sexual fantasy. Currie suggested that he had sex with her dead body. His denial jarred.

"If I killed for necrophilia … I would've maintained the body for more than one day, definitely," he said. He was too distracted, too psychotic, he said. And anyway, if he did, her body would have been stiff, immobile, and would have "come with a lot of struggles".

"So you would find semen all over her body," Marong said.

A prison guard told the jury how Marong likened his crime to "hunting in the wild".

"They're slaves," he allegedly said. "And she met the criteria."

The day after he killed her, with her eyes and mouth open, which "reminds me of the animals that normally get slaughtered at my workplace", he dumped her body on the grass verge of Main Rakaia Rd, near State Highway 1. He doused her with petrol, set her alight, and fled, leaving a cigarette lighter with his DNA on its striking wheel, a blue beanie, also traced back to him, along with the sheep's tongue.

Marong tried to blame a scrambled mind to explain away everything. To get him off. But not even this remarkably inventive and dedicated serial liar could explain the tongue.

"I have no clue. Simply I was disoriented for everything I do in life."