DO no harm. That is the oath taken by medical students as the embark on training to become doctors.

And yet, within a profession whose whole purpose is to save lives there have been a tiny number who have done the opposite: the serial killers who took their patients lives instead.

Their crimes both horrify and fascinate, and the psychopathology of two in particular - Harold Shipman to John Bodkin Adams - will be in the spotlight tonight as medics in Edinburgh gather for a special staging of the acclaimed docu-play, 'Dial Medicine For Murder'.

Read more: Claim Harold Shipman 'would have gone undetected' in Scotland

The one-off event at the Royal College of Physicians in Edinburgh (RCPE) follows on from successful runs at the Edinburgh Festival in 2015 and 2016, and a UK tour, underlining the enduring popularity of the subject.

It was the brainchild of Dr Harry Brünjes, a physician-turned-businessman, and Dr Andrew Johns, a forensic psychiatrist who gave evidence at the Shipman Inquiry following the notorious GP's conviction in 2000 for the murder of 15 elderly patients.

The true victim count is estimated to be even higher, however, at at least 250 - mostly elderly women, but also including a 41-year-old man and probably a child as young as four.

Dr Johns said there was no psychiatric explanation for his crimes - no serious mental illness such as depression or schizophrenia

"The most likely thing was that he had a severe personality disorder with psychopathic and narcissistic traits," said Dr Johns.

"The psychopathic bit meaning he liked to exert astonishing control over his elderly patients and to kill them when he felt they had bothered him enough.

"As for the narcissistic bit, he loved the adulation of his patients. He loved being thought of as the most senior and wise doctor in Hyde [the town in Greater Manchester where he work].

"He shouted at and belittled those that he thought of as his intellectual inferiors.

"I think he became addicted to death and I think he derived a pleasure from being there at the moment of death with his patients. Not quite a sexual pleasure, but rewarding. A thrill."

Read more: Medics 'should not be prosecuted' for mistakes that kill patients 

The show takes the form of a drama-documentary, comparing the background, arrest, trial and legacies of Shipman and Bodkin Adams, followed by a Q&A with the audience.

Of Bodkin Adams, who died in 1983, Dr Johns is less convinced that he was a serial killer with homicidal cravings as opposed to a doctor of lethal incompetence.

Between 1946 and 1956, more than 160 of his patients in the Eastborne area died in suspicious circumstances - of which 132 left him money or items in their wills.

But Dr Johns believes the era - coinciding with the launch of the NHS - also made prosecutors nervous.

"It's a curious case because the Home Office pathologist reckoned there might be 150 victims, but the Attorney General decided to prosecute on four," said Dr Johns.

"But he chose as the first case the only one of the four for which there was no body and no post-mortem, and therefore was relying on documentary records.

"There's a question about why the attorney general botched the prosecution. The feeling was that the Government was not very keen to see Bodkins convicted because the health service was new and it was thought that if a doctor had been sentenced to death it would have led to a mass defection of GPs.

"But at some of our earlier shows we also had some elderly GP colleagues of Bodkin Adams who said during questions, 'you haven't emphasised just how incompetent he was'. I think these days it would be difficult to show that he had the intention to murder.

"I think he might have been done for medical manslaughter, where it could possibly be argued that he was not negligent, but just so bad at his medical practise that folk died."

Read more: More help needed for 'neglected' personality disorders, says Scots psychiatrist

It is estimated that psychopaths make up around one per cent of the population.

These are not people who are mentally ill, but people whose brain structure simply makes them incapable of experiencing normal human emotions such as empathy or guilt.

They are pathological liars, innately selfish, shallow and egotistical.

Most do not turn into serial killers, of course, and although they made up around a quarter of male prisoners many gravitate instead to positions of power in politics, or business.

It is hard to say whether a disproportionate number are attracted into medicine, although studies have shown comparatively high levels of narcissism among surgeons - a trait associated with psychopathy.

Laughing, Dr Johns adds: "That might not be a bad thing. If someone is cutting you open, you want them to believe that they are a jolly good surgeon - you don't want someone with an anxiety disorder, cutting open your abdomen."

Another recurring theme for medical killers is a need for glory. While most serial killers revel in the depravity of their crime, many hospital killers are seek hero status instead.

In the Unites States in 1987, nurse Richard Angelo fought to overcome feelings of inadequacy by injecting patients with a deadly cocktail before coming to their rescue in order to impress colleagues with his life-saving expertise.

Unfortunately, only 12 of his suspected 37 victims survived, and he ended up convicted of murder.

In the UK, the nearest comparison is Beverley Allitt, the so-called 'Angel of Death' who was convicted of murdering four children and attempting to kill three others, often with insulin overdoses, in the children's ward of a Lincolnshire hospital in 1991.

"She had this desire to be seen as a jolly good nurse at responding to paediatric cardiac arrests, which she had actually caused," said Dr Johns. "When you looked at her psychiatric history, there was a history of self-harm, of cutting, of not coping, lots of sickness absence from work.

"This was a needy, unhappy person."

Professor Derek Bell, president of RCPE, said: “This fascination is also not a new phenomenon, and the College’s Heritage team will be displaying some real-life murder weapons from the College’s collection on the evening.

"As uncomfortable as it can be, this College has always attempted to study and investigate these crimes, and I look forward to Dr Brunjes and Dr Johns carrying on that unpalatable, but necessary work at this show while thoroughly entertaining our guests."