The family of Damian Rzeszowski - who killed six people including his wife and children in Jersey in 2011 - has told an inquest he should have been in a psychiatric hospital rather than prison.
Rzeszowski was found hanging in his cell at HMP Full Sutton in March 2018.
He had been serving 30 years for manslaughter on the grounds of diminished responsibility for the stabbings at Victoria Crescent in St Helier on a Sunday afternoon in August following a barbecue.
Rzeszowski killed his wife Izabela( 30), their two children Kinga (5) and Kacper (2), his father-in-law Marek Garstka (56), his wife's friend Marta de la Haye (34) and her five year old daughter Julia, in a frenzied attack.
The 37 year old died in the maximum security prison two weeks after medical staff referred him to Broadmoor special hospital.
A statement from his parents in Poland complained of mistreatment and said their son should have been in a mental hospital, and not a segregation unit, due to his mental health.
Senior coroner Professor Paul Marks told the inquest jury the sentencing judge in Jersey in 2012 had remarked that, unlike in England and Wales, he did not have the power to send a prisoner to a secure mental hospital such as Broadmoor, but had expressed the hope it could be made possible once he was in the UK prison system.
Kevin Brennan , a former psychiatric nurse who worked at Full Sutton at the time of Rzeszowski's incarceration, said he had been subject to a thorough assessment by a consultant psychiatrist when he arrived from Belmarsh Prison in 2013.
The decision was made not to refer him to a secure hospital.
Mr Brennan told Hull Coroner's Court Rzeszowski placed on a programme of regular supervision due to a 'severe and enduring diagnosis of psychosis and depression”.
Signs of improvements led to a scaling down of treatment in 2015.
In January 2018 Rzeszowski self-harmed and tried to overdose. He was placed in segregation under a closer supervision regime.
Mr Brennan said a psychiatrist decided Rzeszowski was showing signs of psychotic illness. When he would not comply with his medication, a referral was made to Broadmoor.
The inquest continues and is expected to last nine days.