Christian Lacey, 21, slaughtered mum Liz, 63, just hours after being held in police custody in April last year.
He was allowed to leave despite having already tried to strangle his dad, Johann Wierzbick, and half-brother, Jacob Carfoot, the previous day.
And after killing Liz, he walked to his nan’s home and tried to kill her carer, Edwina Holden, by stabbing her 12 times, reports Liverpool Echo.
But a review says police, a GP and mental health workers showed “unconscious bias” and “made an assumption” Christian was not dangerous because he was “well dressed and from a nice family”.
Liz’s body was discovered by brother, Ian Wright, following the horrific “manic act of violence”.
Christian’s concerned family had also taken him to A&E at the Royal Liverpool Hospital, and his paranoid schizophrenia was missed.
Before then, he had gone missing to London where he gave his new mobile phone to a homeless man as he thought it had been hacked and he was going to die.
However, a mental health worker noted he was a “pleasant young man who did not appear to have any significant mental health issues”.
They also mistook his symptoms for Asperger’s Syndrome and cannabis-induced psychosis as opposed to schizophrenia.
Christian was initially given a murder and attempted murder charge, but he later agreed to accept pleas on manslaughter on the grounds of diminished responsibility.
He is now a patient at high security Ashworth Hospital in Maghull, after being made the subject of an indefinite hospital order.
Judge Clement Goldstone, QC, has ordered Merseycare NHS Foundation Trust to make a public report into the case.
The report reads: “The review identified that in the panel’s opinion the root cause of the incident was (Lacey’s) complex presentation which was not explored by the practitioners in depth.
“This lack of exploration may have been due to (Lacey’s) presentation as well as a lack of sufficient training in exploring psychopathology.
“This resulted in a delay in diagnosis and therefore untreated psychosis.”