"IF you treat someone for long enough as an animal, they'll turn into an animal."

Robyn Forbes is talking about her former partner, Ewan MacDonald, who was by some distance Scotland's most dangerous prisoner, until he was incarcerated in the state hospital in Carstairs earlier this year after stabbing a warder in the neck at Low Moss Prison outside Bishopbriggs.

MacDonald has a long history. In 2016, then 22, he was sentenced to nine years in prison for attempted murder after repeatedly stabbing another man in an Aberdeen flat. He had 11 previous convictions, nine of them for serious assaults. These continued against officers in prisons throughout Scotland until he was belatedly diagnosed as mentally ill in March and sent to Carstairs.

Forbes blames MacDonald's prison treatment – including lengthy periods in segregation, known in prison slang as "the digger" – for his violence. "These attacks have happened, but they wouldn't have happened but for the negligence of the Scottish Prison Service."

She adds: "They knew, and he knew that he had a mental health problem. The person now is not the real Ewan."

But for a few weeks at liberty, MacDonald has been in prison almost constantly since he was 16, when he was sent to the Polmont Young Offenders Institution. There, Forbes claims, he spent his first spell in segregation, 14 months in total.

Since graduating to adult prison he has been in every Scottish jail, shuttled from one to another on what prisoners call "the ghost train". Since March last year he was again in segregation, effectively solitary confinement. This means that the prisoner does not mix with others, he is fed separately, sometimes through a slot in the cell, there is limited exercise and when he is taken out officers wear full riot gear, including helmets and stab-proof jackets and trousers.

Initially imprisoned at Grampian, he was later sent to Perth, Saughton, Barlinnie, Kilmarnock and, finally, Low Moss. Forbes believes it was this last stint of segregation which sent MacDonald over the edge and into obsessive attacks on staff.

According to prison officers the Sunday Herald has spoken to, MacDonald was moved around because no one prison could cope with him for prolonged periods due to his violence and threats to kill officers, and also to give staff breaks.

"I don't believe prisoners should be kept in luxury with colour TV and games consoles," Forbes says, "but if someone's in a prison's custody they should be looked after, not just left."

The Scottish Prison Service does not comment on individual prisoners, but it denies that there is an insufficient level of care. Forbes, however, believes that rehabilitation is now no longer a part of the prison system.

The most famous attempt to change the ways of the country's most hardened criminals was the Barlinnie Special Unit, which ran from 1973 to 1994, and from which graduated Jimmy Boyle, every bit as violent as MacDonald, but who had his life turned around by discovering art.

Author and ex inmate Johnny Steele said of the unit: “No other prison in Scotland would take me. You’ve got to bear in mind in all those years of fighting, being locked up and rioting you’ve got a coat of armour as a way of surviving. In the special unit, all the armour was off, and through time chip, chip, chipping it a wee bit.

“A lot of the guys who went in there and came out didn’t go back into prison. The unit played a key role in changing the way we tried to reintegrate people into the community and it’s still seen today.”

Forbes says the nature of MacDonald's illness is particularly serious, and adds: "What Ewan does now makes me cry. He's cut everyone off."

She is now trying to find a lawyer to take to take up MacDonald's case. "I just want someone to take responsibility," she says.

The Barlinnie Special Unit was pioneered by prison officer Ken Murray who was one of the most significant advocates of prison reform. It came about because in the early 1970s the Scottish Prison Service was wracked with riots and was struggling to contain a number of exceptionally difficult prisoners, like Jimmy Boyle. Boyle was moved from the cages at Porterfield prison, Inverness, to the new unit at "the Bar-L", as it is often referred to by prisoners and the public. The Porterfield governor signed him off with the belief that Boyle was likely "at any time to attack and kill anybody with whom he is liable to come in contact".

Boyle later recalled: "Murray gave me a pair of scissors to cut the string [on a parcel]. There I was awaiting trial for six attempted murders of prison staff and being given a weapon by one of their colleagues. It was mind-blowing."

Eventually the system turned against Murray and and the unit was allowed to wither away to its closure in 1995.