Dawn McKenzie, 34, bled to death in hospital after being stabbed 10 times at her home in Hamilton, South Lanarkshire in June 2011.
Unknown to Mrs McKenzie, her killer suffered from dissociative state, which left him unable to properly establish what was fact and fiction, after being physically abused by his natural parents at a young age.
Now a significant case review commissioned by Glasgow Child Protection Committee (GCPC) has been published ahead of a fatal accident inquiry (FAI) into her death.
It criticised under-pressure social workers, who did not visit the boy for two months after he had been placed with Mrs McKenzie, for being in breach of their legal duty. The victim worked for agency Foster Care Associates [now Core Assets], which accepted the placement from Glasgow City Council.
The council's social work department and the children's panel who dealt with the case are accused of over-identifying with the plight of the boy's mother, and losing focus on the teenager. But it admitted the tragedy was almost unprecedented and his "murderous attack" could not have been predicted. No disciplinary action has been taken against any staff.
The boy, who cannot be named for legal reasons - he is known as D in the report - was ordered to be detained for seven years at the High Court in Edinburgh. He admitted to the killing, carried out when he was 13 years old.
The nine-strong independent panel found the boy showed no sign "whatsoever of aggression towards adults since he had been accommodated three years previously".
It concludes: "Inadequate staffing, inexperienced staff, shortage of resources and disruption to supervisory and management structures all had an impact, not only at the time but in the longer term."
The report highlights concerns about social work systems and practice prior to D being taken into care, "including workers and children's panel members over-identifying with the mother's plight". This led some of those involved to ignore a fundamental rule that the child's needs should be paramount in any such cases, it says.
The panel adds: "A somewhat more cautious inerpretation of D's progress might have been helpful in sensitising them to the possible risks he presented, albeit that his extreme violence was unforeseeable."
While stating that such an attack by a child is almost unheard of, the panel concluded that staff and previous carers had thought the boy was flourishing in his placements, and was "looking forward happily and hopefully to a positive future". However, it emerged in the court case that he had a troubled background involving significant abuse.
The report suggests social workers must be alert not just to child protection cases which are going badly, but also those which are going unexpectedly well. "The fact he [D] appeared relatively unscathed should probably have rung alarm bells," it says.
Donald Urquhart, independent chairman of the GCPC, said that the case while "undoubtedly tragic, was in no way foreseeable." He added: "As far as the committee understands, there have been no obvious cases like this elsewhere."
The full report and accompanying background papers will be provided to both the Care Inspectorate and the Crown Office for the FAI.
Ruth Stark MBE, a spokeswoman for the Scottish Association of Social Workers, said the case was complex, but the report highlighted that "working with young people and those who care for them, the social workers need to be supported by their employers."
She added: "They [the panel] describe people who are skilled and able to do life-changing work. But they acknowledge that this can only be done when the social workers have time to use their skills."
A council spokesman said: "This is an incredibly tragic case and we are sorry that Mrs McKenzie and her family suffered this ordeal.
"As was previously said in court and has been established through the significant case review, there was nothing remarkable in the boy's behaviour in the lead-up to this attack."
The spokesman described references to understaffing and a lack of time for reflection for social workers in the report as historic, adding that staffing had been "significantly enhanced".