The decision by a teenager to stab his grandmother 17 times while she was in the bath was “not the reaction of a normal person who is angry”, a court has heard.
Dr Peter Misch, a forensic psychiatrist, told a jury in Lewes Crown Court that Pietro Addis, 19, had committed an “extraordinary killing” but that he had been driven by a delusional psychotic belief.
Dr Misch, instructed by the defence, said that while Addis told him the killing wasn’t planned, “things had built up” in the weeks prior to the attack.
The psychiatrist said: “Addis went into the bathroom in extraordinary circumstances and committed an extraordinary killing. That's not the reaction of a normal person who is angry.
"He was able to function in most ways, but he was driven by a delusional belief.
"He was driven by the fear, that became overwhelming, that he was under threat."
The jury previously heard from Dr Misch that he was told by Addis that he thought his grandmother, Brighton restaurateur Sue Addis, was “going to kill me”.
Dr Misch added: "In his mind it was the delusional belief that his grandmother was threatening him in a very serious way.
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"It became delusional in the sense that he could not rationalise his way out of it."
While being cross-examined by the prosecution, Dr Misch was asked whether Addis hitting a wall could be to “dissipate anger”, which he agreed with but added that anger was a “normal emotion”.
Dr Misch said: "It can dissipate a number of emotions. The point is anger is a normal emotion, most people experience it."
The psychiatrist said Addis was harmfully using drugs in the months before the offence.
The court heard that Addis regularly smoked cannabis but that his use significantly decreased prior to the incident.
Addis is accused of murdering his grandmother at her home in Cedar Gardens, Brighton, in January 2021, when he was 17. He has pleaded guilty to manslaughter but denies murder.
Mrs Addis, 69, ran well-known Italian restaurants Donatello in The Lanes and Pinocchio in New Road.
The trial, being presided over by Judge Christine Laing KC, continues.
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