The man accused of murdering Christchurch prostitute Renee Duckmanton told a psychologist who concluded he was mentally fit to stand trial that sex workers were "dirty, immoral and inferior", a court heard today.

Sainey Marong, a 33-year-old Gambian-born butcher, yesterday admitted at the High Court in Christchurch that he strangled Duckmanton in May 2016 and set her body on fire.

But he denied that he meant to kill her, 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.

Yesterday he spent a dramatic day on the witness stand to repeatedly explain that he was "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".

In response to his evidence, the Crown was granted permission to call rebuttal evidence of a psychiatrist and a psychologist who assessed Marong before he stood trial.

Both experts concluded that he did not meet the criteria for a defence of insanity.

Today, 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. He had never come to the attention of mental health services before to his arrest.

Metoui found Marong to be prone to histrionic behaviour with an ego-centric personality that included a considerable sense of entitlement and grandiosity.

There was also a high-propensity to deceive and manipulate others, Metoui told the court.

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".

"It is my opinion that Mr Marong has a severe pathological personality structure and endorses a constellation of traits consistent with both anti-social and narcissistic personality disorder," Matoui said.

And he concluded: "It is my opinion that Mr Marong does not suffer from any underlying psychiatric disorder that would constitute mental impairment in terms of the Criminal Procedures Mentally Impaired Persons Act 2003."

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Metoui was in court yesterday to witness Marong's evidence. Asked by Crown prosecutor Pip Currie if he saw anything that gave him pause to change his opinion, Metoui said that, if anything, it reinforced his opinion.

The only difference in Marong from when he saw him was that he cried during his evidence yesterday, Metoui said.

Marong admits picking up 22-year-old Duckmanton from Christchurch's red light district on May 14, 2016 and engaging her services for sex.

Somewhere around Templeton, just south of Christchurch, they stopped on the roadside and had sex in the backseat of his car, Marong said.

Afterwards, Duckmanton wanted to return to Manchester St, saying the "job is done".

But Marong wanted to continue to Rolleston where he was fixated on renting a property, and he says there was an argument.

"She started yelling at me," Marong said.

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.

Asked by defence counsel Jonathan Krebs if he meant to kill her, Marong replied, "I have no motive to cause any harm, I just did it to silence her."

The Crown claims its case against the butcher is "overwhelming".

It claims DNA samples taken from Duckmanton, and from samples found where her body was dumped, allegedly belong to Marong.

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

The trial, before Justice Cameron Mander, continues tomorrow.