Edgington killed her mother at her home in Forest Row, East Sussex, in 2005.
BBC: Nicola Edgington: 'Checks would not have stopped murder'
The prosecution said Ms Edgington claimed her responsibility was diminished because of her mental state.
Kerry Clark, 24, said she grabbed the knife Nicola Edgington threatened her with in Bexleyheath, south-east London.
Nicola Edgington, 32, of Greenwich, virtually decapitated Sally Hodkin, 58, six years after killing her own mother.
On that occasion, Edgington pleaded guilty to manslaughter due to diminished responsibility, a plea accepted by the prosecution.
BBC: Nicola Edgington: 'Checks would not have stopped murder'
Edgington was treated as an inpatient in a medium secure psychiatric facility by the Oxleas NHS Foundation Trust.
"However, as the IPCC investigation concluded, whilst Edgington was vulnerable, she was compliant and herself seeking medical assistance, " he said.
BBC: Nicola Edgington: 'Checks would not have stopped murder'
This year's Old Bailey trial heard that in the earlier case Edgington initially said she did not kill her mother.
On that occasion Edgington pleaded guilty to manslaughter due to diminished responsibility, a plea that was accepted by the prosecution.
Ms Edgington was sent to a secure hospital after admitting killing her mother, but was released into community care in 2009.
And Edgington had killed before - in 2005 she stabbed her mother to death at her home in Forest Row, East Sussex.
Ms Edgington, 32, of Greenwich, south-east London, denies murder and attempted murder.
After the verdict, Mrs Hodkin's family said they did not understand "how or why" Edgington had been allowed back into the community.
BBC: Eleven mental health patients in care of Trust have killed
Nicola Edgington, from Greenwich, south-east London, was found guilty of murdering Sally Hodkin, 58, and the attempted murder of Kerry Clark, then 22.
The Old Bailey judge also gave Edgington a minimum sentence of 20 years, which will run concurrently, for the attempted murder of Kerry Clark, 22.
The court heard that in the weeks leading up to the attack, Edgington had a miscarriage and had received abusive messages from a former boyfriend.
Its chief executive, Stephen Firn, said it was "a matter of extreme regret" that Edgington was able to leave the hospital on the morning of the stabbings.
David Edgington for his work for the industrial heritage of the stationary engine and Lady Judy Martin, who is the director of the Young Persons Concert Foundation.
Cdr Basu added that when officers took Edgington to the Queen Elizabeth Hospital in Woolwich, they were told that hospital security "had the matter in hand and that there was no need for police attendance".
BBC: Nicola Edgington: 'Checks would not have stopped murder'
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