Peter Manuel, Scotland's most notorious serial killer, was a violent sex offender who turned to murder to satisfy his increasingly depraved urges.

However, a new book claims that he suffered from a severe sexual disorder and may not have carried out the rape that predated his spree of violence.

Manuel was the third-last person to be hanged in Scotland when he was executed at Barlinnie Prison in July 1958.

Allan Nichol, author of Manuel: Scotland's First Serial Killer, now believes that Manuel suffered from paraphilia - the need for abnormal stimulus to achieve full sexual gratification. In Manuel's case he was aroused by the fear felt by his victims.

Consequently, Mr Nichol, one of the country's leading advocates, says Manuel was incapable of the rape for which he was convicted in 1946.

“That conviction represents the only time he was charged with actual rape and it has to be questioned,” Mr Nichol said. “For physical and psychological reasons he found it difficult to achieve full sexual relations.

“The clear implication is that he achieved sexual relief through violence and required a stronger stimulus after each attack. He achieved his satisfaction in other ways.”

He added: “I'm out on a limb, but I think attempted rape would have been the correct charge.”

Although the 1946 victim believed she had been raped she was dazed after being brutally beaten by Manuel. She had also had a hysterectomy, which could have accounted for her internal pain.

A medical examination of the woman revealed no sexual injury or trace of Manuel's semen. However, semen was found on his clothes, adding to the theory that he did not penetrate his victim.

“Complete sperm were found on his trousers, shirt and singlet. His dark secret meant he did not actually commit the full crime. He would have raped had he been capable,” said Mr Nichol.

Manuel, who was born in the United States but moved to England with his family at the age of six, received a prison sentence of eight years for the rape. When he was released in 1953 he moved to Scotland, where he soon returned to violence. Between 1956 and 1958 he killed at least seven times, although it is believed that the true figure was higher. Whilst awaiting execution at Barlinnie Prison in 1958, he is said to have admitted killing up to 18 people.

Manuel was found guilty in May 1958 of murdering Marion and Vivienne Watt, a mother and daughter, along with Mrs Watt's sister, Margaret Brown, by shooting them in the head at close range at a house in Burnside, Glasgow. He was also convicted of murdering the Smarts, a family of three, in Uddingston, Lanarkshire, as they slept, and the 17-year-old Isabelle Cooke as she walked to a school dance.

When detectives later took Manuel to the field where he buried Isabelle and asked where her body was, he replied: “I'm standing on her now.”

Mr Nichol said that uncovering the truth about Manuel earlier would have been unlikely to prevent his terrible crimes.

“I don't think there was a mistake made, it was just a technicality,” he said. “He would still have received a long sentence for attempted rape.

“It would not have made any difference to the way he was going.”

Manuel: Scotland's First Serial Killer is published by Black And White Publishing

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