Benjamin Hoffmann pleaded guilty mid-trial in November to intentionally killing Hassan Baydoun, 33, Michael Sisois, 57, and Rob Courtney, 52, and the manslaughter of 75-year-old Nigel Hellings on June 4, 2019.

His Northern Territory Supreme Court trial heard the 48-year-old was "hunting" a man named Alex Deligiannis, who the court heard was a drug dealer and pimp.

But on Friday at Hoffmann's sentence hearing, psychiatrist Siva Bala said Hoffmann told him that his ex-girlfriend Kelly Collins had been taken hostage and he had been searching for her during his drug-fuelled rampage.

"That's one of his reasons, the other was to stop this control that he believed Alex had over Kelly. He was pimping her out ... and associates of Alex or whoever had raped her," Dr Bala said.

Dr Bala agreed with prosecutor Lloyd Babb that Hoffmann had a "serious grudge" against Mr Deligiannis, and found he was delusional and suffering drug-induced psychosis.

"Probably towards the end of May, around the time he called the police to his house and said there were people in the roof," Dr Bala said when asked when the psychosis started.

"It's a very complex case."

Expert evidence about Hoffmann's mental health and state when he killed his four victims is likely to determine the severity of his sentence.

Dr Bala said Hoffmann's psychosis was not usual and shortlived but he disagreed with Mr Babb that it was more likely to be drug intoxication. However, he agreed it was possible Hoffmann could have hatched the plan to find Mr Deligiannis before he became mentally ill.

"My opinion that he was psychotic is on the balance of probability. It is not without doubt," he said.

"As he started using methamphetamine in April, May ... his delusions crystalised against Alex ... became more and more encompassing involving other people, including Ms Collins."

Psychiatrist Alan Jager said Hoffmann told him he believed he was in danger and it was "him or them and he decided to commit the offences against the people he believed were causing him harm".

He reported Hoffmann had been unable to understand his conduct was wrong soon after the incidents. However, the court heard he was observed not to be psychotic the day after.

"It's certainly in favour of it being a drug-induced psychosis versus a constitutional psychosis, such as schizophrenia," he said.

"He was smoking it heavily then he shoots it up on the morning of the incident ... These things aren't consistent ... We can have moments of clarity and then psychotic thought again."

He said Hoffmann may have become psychotic as early as a month before the killings based on his mother's report that Hoffmann had been "talking in riddles" when he took her to a cafe on Mothers Day.

Asked if Hoffmann could have been feigning psychosis, Dr Jager said it was possible to mimic the symptoms but hard to sustain the ruse.

"On the balance of the facts that I saw there was evidence that he was psychotic," he said.

Psychiatrist Lester Walton agreed, saying "the main feature was deluded ideas".

"He was convinced that he needed to rescue Kelly, and that she was in danger and that was driving him," he said.

"He was concerned about her welfare, she was being raped and prostituted in order to provide funds for drugs."

Mr Deligiannis has previously denied he was a pimp and a drug supplier.

The case will return to court on July 25 for further sentencing submissions.

Australian Associated Press