Mark Jarvie killed Jane Kelly, 22, in the bedroom of his family home in Powmill, Kinross, on 28 October last year.
His parents, Gordon and Linda, walked into his room to find Miss Kelly had been strangled and stabbed in the head with a knife.
The art student was lying face down on the bed fully clothed. Jarvie was sitting on the floor naked with his hands round her neck. Jarvie’s mother called paramedics, but nothing could be done to save Miss Kelly.
Jarvie, 22, went on trial at the High Court in Aberdeen yesterday, charged with murdering Miss Kelly.
However, the judge instructed the jury to find Jarvie, who has been detained at Carstairs, not guilty of the charge on account of his insanity.
A joint minute read out to the jury stated that it was agreed that Jarvie, who has been diagnosed with schizophrenia, had killed Miss Kelly.
But psychiatrists had concluded that he had suffered a “total alienation of reason” at the time of the offence.
The court heard Jarvie’s friends and family had become concerned about his aggressive and out-of-character behaviour in the weeks leading up to his girlfriend’s death.
After the hearing, Miss Kelly’s parents, Graeme, 49, and Karen, 47, of St Andrews, said they were struggling to understand why their daughter was not informed of his condition.
Mr Kelly, a solicitor from Edinburgh, said: “Jane was the greatest and most generous of girls and her loss causes us overwhelming pain.
“We, as a family, continue to grieve. We cannot come to terms with why Mark Jarvie perpetuated such a wicked, pointless and senseless act.
“We have listened to all the psychiatrists’ evidence and we are being asked to accept that Jarvie was, and remains, very ill.
“Nevertheless, we will never understand why he did what he did, nor can we forgive.”
Mr Kelly said the family still had many questions that remained unanswered. He said he hoped they would one day learn the truth about why their daughter had died.
He added: “There were concerns about him in the days leading up to this – why was Jane in that situation? Why was she exposed to such danger when people had such concerns?”
Miss Kelly was thought to have been dating Jarvie for about eight months. She was studying fine art at Duncan of Jordanstone College of Art and Design and was living in Dundee.
She had travelled by train to Inverkeithing station where she met Jarvie the day before she was killed.
Earlier that day, his parents had cancelled their plans to travel to Thailand on holiday. Their son had called them not long after they arrived at Edinburgh airport and told them he had “gone a bit crazy” and cut his leg.They drove home and arranged for him to see a doctor.
Dr Gyda Meeten concluded Jarvie was in need of a medical assessment but did not meet the criteria for detention under mental health legislation.
He met his girlfriend at the train station shortly afterwards and she was dead the following day.
Jarvie was detained indefinitely at Carstairs Hospital yesterday. The judge also imposed a restriction order.