The man accused of murdering Claire Turnbull may have battered her to death with a hammer because he saw his girlfriend was frightened by her advances, a jury heard today.

Psychiatric experts said the “fear and helplessness” Aaron Donald claimed he saw in Laura McMurdo’s eyes could have triggered a violent outburst caused by Post-Traumatic Stress Disorder (PTSD).

Donald, 28, admits killing 36-year-old Claire at the flat he shared with McMurdo in Blairhall on October 5 last year.

However, he denies murder claiming he was suffering from a mental disorder and “diminished responsibility” at the time.

The High Court at Livingston today heard conflicting evidence from expert medical witnesses on day nine of the trial.

Dr Ali Ajaz, 38, a consultant forensic psychiatrist for the East London NHS Trust, said he had at first diagnosed Donald as suffering from a personality disorder, but not any psychotic illness.

At a second interview earlier this month he said the accused had described being forced to write a suicide note and having a knotted rope put around his neck in Perth Prison before allegedly being forced to perform sex acts on prison officers and other inmates.

Donald claimed the sexual abuse had continued on repeated occasions for the following three months and told the psychiatrist: “It was a horrific what happened, but I couldn’t help it.”

Dr Aziz explained to the jury that systematic sexual abuse had the potential to be a highly significant factor in the death of the victim.

“It could be argued that this is evidence of genuine reaction to genuine traumatic events ... or on his remand he’s been able to research the symptoms of PTSD.”

He suggested that Donald could have compared his partner’s “fear and helplessness” at being subjected to a sexual assault to his own emotional experience, triggering his “lethal” outburst of violence against Claire.

Under cross examination by advocate depute Bernard Ablett, Dr Aziz agreed “absolutely” that it was a matter for the jury whether they believed Donald’s uncorroborated story of being sexually abused in prison.

Dr Aziz said: “This a man who has a history of violence, a history of using weapons but no history of killing. Part of my role is to establish if the killing may have happened as a result of his mental state. My opinion was that there was no psychotic illness.

“It was my view that he had responsibility (for his actions) at the time of the killing from my assessment and information available to me.

“The assessment of his mental health at the time did not suggest his mental state was so abnormal that he wasn’t able to understand what he was doing.”

Professor Matthias Schwannauer, 49, professor of clinical psychology at the University of Edinburgh, said Donald’s psychiatric history revealed he suffered from a range of personality disorders and had repeatedly presented with psychosis related symptoms including hallucinations and paranoid delusions.

He said Donald had suffered from depression, anxiety, PTSD and psychosis for most of his adult life. He had undergone mental health treatment since he was 10-years-old and had been suffering from PTSD following the traumatic death of a close friend when he was 12 years old.

He had started using substances to self-medicate and developed long standing substance abuse problem.

Donald also complained of frequent memory intrusions, nightmares, flashbacks and other symptoms of post-traumatic stress which could be linked to physical and sexual assault, trauma related to violence, and fear for his life and personal safety.

Professor Schwannauer told the jury: “In my opinion it is highly likely at the time of the offence he was suffering from acute psychosis and post traumatic distress. It is also highly likely he was not in receipt of effective dose of anti-psychotic medication.”

He said the incident with Claire in which he perceived his partner to be at risk of sexual assault was “highly likely” to trigger a reaction, adding that Donald’s mental and emotional state may have significantly influenced his behaviour at time of Claire’s death.

The trial, before Lady Scott, continues.