A TEENAGER who denies murdering his grandmother as she lay naked in the bath has been described in court as an “unreliable historian”.

Pietro Addis, 19, told a psychiatrist instructed by the prosecution that he heard the voice of rapper Biggie Smalls in his head before stabbing Sue Addis in the bath 17 times at her home in Brighton on January 7 last year.

He also told the psychiatrist that in the lead up to the killing he believed the FBI were following him and that people were watching him through Instagram.

Another psychiatrist, instructed by the teenager's defence team, has told the court “the frenzied nature of the attack on Mrs Addis is consistent with a psychotically driven homicide” and that his “responsibility for killing his grandmother is diminished because of that abnormality of mind”.

Dr Duncan Harding, a consultant child and adolescent forensic psychiatrist instructed on behalf of the prosecution, told the jury yesterday that he had a “two-to-three hour” face-to-face interview with the teenager in Bluebird House, a secure mental health facility, on June 1 last year.

Prior to being transferred there, the teenager had been remanded in custody at HMP Cookham Wood.

During the interview, Pietro claimed that the guards at the prison were “trying to kill me” and inmates were shouting “go and hang yourself”.

The psychiatrist told the jury that Pietro’s narrative regarding events in his grandmother’s house leading up to the stabbing often changed during the interview.

He said the teenager told him he did not keep a knife in his bedroom, “then he claimed he kept one to defend himself [from his grandmother]”.

“Then [later in the interview] he said he had a knife in the bedroom to cut chorizo,” said Dr Harding.

Dr Peter Misch, instructed by the defence, told the court on May 4 that he also interviewed Pietro Addis several times following the stabbing, during which the teenager told the doctor he believed that his grandmother was “trying to kill him”.

However, Dr Harding told the jury that when the teenager told him about the incident with his grandmother, he thought that Pietro Addis was “making this up”.

“I formed the view that he was being dishonest,” Dr Harding told the court, before adding: “I found him to be an unreliable historian.”

Dr Harding also said that, in his opinion, there was "no evidence of psychotic disorder" in a dramatic 999 call Pietro made to the police after he stabbed Sue Addis, or in the police bodycam footage taken by an officer after the emergency services later arrived on the scene.

Pietro Addis has admitted killing his grandmother but denies murder.

The trial at Lewes Crown Court continues.

READ MORE: Sue Addis murder trial: Recap from Lewes Crown Court