Mental health staff treating Alexander Palmer warned police about him more than once before he killed a dog walker last year.
Palmer, a former Royal Marine, suffered with psychosis which got worse in 2017 when he stopped taking his medication.
He had recently bought a hunting knife and machete, according to his medical notes, and had told health professionals about his desire to harm dog walkers since at least 2015.
“He said he would tie them to a fence and cut them open,” according to his medical notes.
Palmer received treatment for three years, both while still in the Marines and after being discharged.
But by the time he murdered Peter Wrighton in woods near East Harling last August, he had stopped taking his medication and stopped his treatment.
Mr Wrighton’s children said their discoveries about his mental health treatment left them “shocked, astounded and angered”.
They have questioned how he was able to discharge himself and said mental health professionals had failed him, his family and their family.
The trial at Nottingham Crown Court heard Palmer came off his antipsychotic medication because he “missed” the voices in his head.
In a note he is believed to have written to himself in early 2016, he said: “I took myself off medication after a year or so.
“I said it was because I didn’t need them any more that the hallucinations had gone. But the real reason was because I missed Alex [the auditory hallucination voice]
“I missed the pleasurable agony of someone in my ear, placing his views where mine should be.”
He received treatment from mental health service the Norfolk and Suffolk Foundation Trust (NSFT).
And an NSFT spokesman said staff raised concerns with police “on more than one occasion”.
“It should be remembered that this has been an extreme and rare situation,” they added.
A serious case review is now being held to see if any lessons can be learned from the authorities dealt with Palmer.
South West Norfolk MP Elizabeth Truss said: “Where serious failings have occurred, I hope a very thorough case review will take place and decisive action taken.”
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