The father of a teenager stabbed and beaten to death by two friends in a dispute over a bottle of aftershave has demanded they are never released from prison after being found guilty of the first murder on the Western Isles in 43 years.

Johnathan MacKinnon and Stefan Millar, both 22, betrayed the friendship of Liam Aitchison, 16, from South Uist, after a night of drinking. They pole-axed him with a beer bottle at a derelict house near Stornoway airport before stabbing him 20 times and beating his body.

They were convicted after a three week trial at the High Court in Glasgow. Afterwards, Mr Aitchison's father, Norrie, said they could finally grieve for their son – who would have turned 18 this month – for the first time since his death in November 2011.

With his wife Claire, Mr Aitchison said: "He didn't stand a chance against the violence he came against that night. He was left stripped of his clothes with 20 stab wounds. No family should have to endure what we have.

"The cowards who robbed our Liam of his life and his future should never be able to see the outside of a prison ever again."

Mr Aitchison described their son as their "beloved crazy diamond" who was outgoing and had a heart of gold. The murder, the first since an elderly woman was found dead in 1968, had shocked the community.

Police said MacKinnon, of Stornoway, and Millar, of Carloway on Lewis, have shown no remorse.

Mr Aitchison said the death had left a void in the boy's grand- mother's heart, had a devastating impact on his siblings, and that the family thought of him "every hour of every day".

The trial heard how the three youngsters had been drinking and listening to music at MacKinnon's house but left when his mother got fed up of the noise.

MacKinnon had lashed out at Mr Aitchison when a bottle of his aftershave fell out of the victim's pocket.

The men then made their way to the disused building in Steinish, where Mr Aitchison was attacked and suffered a fractured jaw and nose. Two major arteries were struck which a pathologist said would have been fatal.

The killers clambered out of the blood-soaked building without leaving any significant trace they had been there. They returned to MacKinnon's home and concocted a story to cover-up what they had done.

They claimed after a night of socialising they saw Mr Aitchison walk off towards a friend's house.

Shouts of "yes" and "pair of monsters" were yelled from the family when the verdicts were returned. The pair now face life sentences when they are sentenced later this month at the High Court in Edinburgh.

Judge Lord Kinclaven remanded them in custody. MacKinnon has previous convictions for fire-raising, breach of the peace and assault and Millar has a record and was subject to an electronic tag at the timeof the killing.

Detective Inspector Andy Logan of Police Scotland, who led the murder inquiry, said: "Liam was a 16-year-old boy who had his whole life in front of him and enjoyed the support of a small number of persons locally.

"Unfortunately, Liam fell into the company of Johnathan MacKinnon and Stefan Millar, believing them to be his friends and placing his trust in them. They completely betrayed this trust, and in a most brutal and callous way they killed him in a senseless and sustained attack."

l Mr Aitchison's mother has appealed for the return of her purse, containing treasured photographs of her son, after it was stolen by a female outside Pollok Health Centre in Glasgow during the trial.