Alesha MacPhail's killer frequently smoked cannabis

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Her killer, the "evil and wicked" Isle of Bute schoolboy Aaron Campbell, was an extensive user of the drug and had even bought it from his six-year-old victim’s father. Detectives are at a loss to explain the teenager’s motives, although it was just the latest heinous and seemingly inexplicable crime committed by a perpetrator who frequently smoked cannabis. Despite the tolerant public view of the Class B substance, as well as its apparent medicinal benefits, there is a growing body of evidence linking the drug to violent crime and teenage psychosis.

Campaigner Ross Grainger has compiled a “catalogue of suicide and psychopathic violence committed by cannabis smokers in the UK and Ireland” over the past two decades including 200 murders, rapes and savage assaults. He said: “In this case, as in all such cases, I do not say that cannabis 'caused' the perpetrator to do what he did, but rather that it would not have happened if he had not smoked cannabis. “There is copious evidence, going back decades, of the immense harm cannabis can do to an adolescent mind, and it is, in my view, the only possible explanation for this young man's depravity and savagery.” The court heard that one possible explanation was Campbell's dispute with Alesha’s father, Robert MacPhail, over an unpaid drug debt of just £10.

Campbell's only connection to the family was buying cannabis from the victim's father

Mr Grainger said: “This may have given him a grudge. But many people have grudges and are full of bitterness. To act on this in the way he did, inflicting 117 injuries on the girl, requires a warped mind, and in my view only cannabis could have damaged it so. “Abusing and raping children is not unique to cannabis smokers, but when I read of the 117 injuries he inflicted, I knew there must be cannabis behind it; a sustained, frenzied, brutal and psychopathic murder of this kind nearly always does. “Cases involving more than 100 stab wounds are far from uncommon. In one of the cases I've compiled, a man stabbed another man 143 times because he thought he was the devil.” Mr Grainger, a writer from Bristol, has submitted a petition to the UK Government calling for a review into the links between cannabis and violence which has been signed by more than 12,000 people.

He launched his campaign after Becky Watts was murdered by her step-brother Nathan Watts, a regular cannabis user, in his home city almost four years ago. “Scotland is no stranger to this type of murder," added Mr Grainger. "There was the Jodi Jones murder, and the Anne Nicoll murder, which happened within about a year of each other, in which young men deranged by heavy cannabis smoking committed heinous acts of murder. "As with Alesha MacPhail, both cases are notorious for the savagery involved, and for their seemingly inexplicable nature.” Drugs expert Professor Neil McKeganey, director of the Centre for Substance Use Research in Glasgow, said Alesha's murder was "shocking in every respect except one - the killer had a history of extensive cannabis use".