A review of 10 killings - including the road rage stabbing of a pensioner - has uncovered failings at a mental health trust.
Experts examined the deaths of 10 people since 2007, all of which were linked to Sussex Partnership NHS Foundation Trust.
The review found that the Trust did not always learn fully from previous mistakes and sometimes "severely underestimated" risks posed by its mental health patients.
Most recently, Mathew Daley, 35, was jailed for ten years over the death of pensioner Don Lock, 79, who he repeatedly stabbed.
At Daley's trial, his mother claimed she had "pleaded" for her son to be sectioned after his mental health broke down - but her fears were not heeded by mental health staff.
Sussex Foundation Trust apologised, saying their care of him "should have been better".
Joseph Goswell's father killed his mother, Susan, in 2007 after being released from psychiatric care. He said the failure of the Trust was "unbelievable".
He said: "They've allowed my father to go out and murder my mother. If they had stepped in then my mother certainly would never have died that day."
Among the shortcomings at the Trust found by the review included a failure to include the views of families, some who demanded help.
Another finding was that the Trust had failed to always send people with signs of psychosis to specialist services.
Commissioned by the Trust and NHS England, the review looked to see if any lessons could be learned.
In two cases, experts concluded the deaths could have been prevented - although it did not say which ones.
Investigators found that in seven of the killings there was criticism of how the Trust assessed the risk posed by its patients.
In several cases, the process was reported to be "inadequate and the risk posed by the service user went unrecognised or was severely underestimated".
At other times, "risks assessments were not completed or were completed incorrectly" and "risk management plans were not completed".
Investigators also found that assessments were not updated when circumstances changed - such as a new criminal conviction - while some assessments were started but not completed.
"Sometimes service users made threats to kill others but no further action, for example informing the police or warning the person threatened, was taken", the report said
A "think family" approach was rarely, if ever, followed and several of the people who went on to kill might have had a dual diagnosis - such as both a mental illness and a substance misuse problem - but this was not identified.
The report said learning after each killing was not always taken up across the trust and there was some "repetition" in the recommendations made after each one.
Investigators also found that, as recently as December 2015, records were not always updated.
Colm Donaghy, chief executive of Sussex Partnership, offered his "sincere apology and condolences" to families.
"We commissioned this review with NHS England because we want to make sure we have done everything possible in response to these tragic incidents", he said.
He added: "This review sends us a strong message about the need to identify and embed learning when things go wrong in a way that changes clinical practice and behaviour".
List of killings reviewed:
Matthew Daley killed motorist Donald Lock by stabbing him 39 times on the A24 in Findon, West Sussex, in July 2015.
Kayden Smith killed Danish tourist Jan Jansen at his flat in Hassocks in August 2012
David Sole murdered Jonathan Ellison in his Brighton flat in 2011.
Graeme Morris killed his mother and battered his father in South Ayrshire in 2012.
Christopher Jeffrey-Shaw killed Janet Muller. She was burned to death in the boot of a car at Ifield Golf Club, near Horsham, in 2015
Sean Iran was jailed for life for killing Stefan Welch and burning his body on a Brighton golf course in 2010.
Steven Dunne killed Gordon Stalker in Brighton, claiming he was a witch who had captured his soul 2010.
Roger Goswell killed his wife Susan in West Chiltington in 2007 after being released from psychiatric care.
Shane Noble and another man punched and kicked Chris Poole to death outside a shop in Eastbourne in 2012.
Oliver Parsons murdered Joe Lewis on Christmas Day at a flat in Brighton in 2014.