A teenager who claimed paranoid delusions caused him to stab his grandmother to death in a frenzied knife attack was not experiencing psychosis at the time of the killing, an expert psychiatrist told a jury.

Pietro Addis, now aged 19, killed his grandmother Sue Addis as she lay in the bath at her home in Cedars Gardens, Brighton, in a fit of anger, Dr Duncan Harding said.

A melting pot of factors including feelings of entitlement and anger led to the brutal killing of the 69-year-old restaurateur on January 7, 2021, the child and adolescent and adult psychiatrist said.

During an interview with him, Dr Harding said the teenager was inconsistent, relaxed and calm when talking about the belief his grandmother, who ran Donatello restaurant in Brighton, wanted to kill him.

Dr Harding told Lewes Crown Court that the “tragic killing” happened in the context of anger.

The court previously heard from Mrs Addis’s postmortem report that she was stabbed 17 times.

Addis sliced completely through one of his grandmother’s ribs during the attack, a pathologist told the court.

Addis, then aged 17, has admitted manslaughter but denies murder.

The court heard he was told to move in with his grandmother just before Christmas 2020 as arguments between him, his father and stepmother escalated over his ADHD medication, which they had confiscated.

Addis told Dr Harding the ADHD medicine made him feel less paranoid and anxious.

Dr Harding told the court that in December 2020 Addis felt "really low".

"I didn't leave my house because of paranoia," Addis told him.

"I moved in with my gran after Christmas.

"My parents had had enough of me and told me to get out."

Dr Harding said Addis told him his ADHD medication elvanse made him feel better in the short term, but after completing his GCSEs he was dependant on it to do anything.

Dr Harding said Addis felt less paranoid with the medication.

Dr Harding said the drug would not have any effect on psychotic delusions and this showed he was not experiencing a mental illness.

“He had been abusing drugs for many months prior to the killing including ADHD medicine,” he said.

“This was causing great concern to family who refused his access.”

The expert said he saw no evidence of ADHD, psychosis, delusions or obsessions when he interviewed him at a secure hospital.

Addis was “glib” and “unconvincing” when he described fears his grandmother was “going to kill him”, Dr Harding said.

“Describing thinking his grandmother wanted to kill him didn’t sound authentic to me,” he said.

Pietro Addis was not given any anti-psychotic drugs during his time in hospital and was not prescribed any when he was returned to prison, the court heard.

Staff at Bluebird House did not think Pietro Addis suffered from ADHD.

Dr Harding interviewed him in June, six months after the killing, before he was discharged from the mental health facility for young people back into prison.

“At that point, I really felt he was malingering, saying something which he knows is not true in interview,” said Dr Harding.

“A person who knows he is dishonest.”

The expert said he thought the accounts offered by Addis were unreliable.

“He did not act in a way consistent with this reported fear his grandmother was going to kill him.

“He did not report it to anyone else or take steps to protect himself.

“He could have left the house at any time or he could have called the police.

“At the time he was not suffering from an abnormality of mental functioning.”

The doctor summarised what he felt led to Addis killing his grandmother.

“Seeking drugs the family were withholding, escalating arguments within the family, his grandmother being the matriarch and her searching that night for possible hospital admission for Pietro,” he said.

Killing his grandmother in the bath showed a degree of planning, Dr Harding said.

“He would have had to take the knives to the bathroom.

“This provides evidence for a significant degree of planning.

“Not a frenzied psychotic killing as suggested by the defence.”

Pietro dialled 999 after changing his clothes and gave details to police.

“In my view, it is extremely unlikely he would have had the presence of mind to call police,” Dr Harding said.

“Had it been a psychotic breakdown, how would he have known it was a murder and how would he have known his grandmother was dead?”

Tests on his blood and hair showed Addis had taken cannabis, cocaine, ketamine, Xanax and his grandmother’s cancer medication in the months before the killing.

Experts have agreed Addis was not experiencing drug induced psychosis at the time of the killing, the court heard.

Dr Harding’s evidence contradicts what another psychiatrist told the court the day before.

Dr Peter Misch said he believed Addis experienced a “psychotic episode which resolved without treatment".

Dr Misch said he believes he was driven by a temporary, transient paranoid delusion.

“I think he had a brief psychotic breakdown based on the delusional belief that his grandmother meant him some harm,” he told the jury.

The trial continues next week.