A review of 10 killings - including that of a pensioner who was stabbed after a collision between two cars - has uncovered failings at a mental health trust.
Sussex Partnership NHS Foundation Trust has already apologised for its role in the lead up to the death of Donald Lock, who was stabbed 39 times on the A24 in Findon in July 2015.
But a new independent review has found the trust did not always learn fully from previous mistakes and sometimes "severely underestimated" the risk posed by mental health patients.
It also failed to include the views of families, some of whom pleaded for help, and did not always send people with signs of psychosis to specialist services.
Commissioned by the trust and NHS England, the review analysed previous reports into the 10 killings 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. Neither of them was the case involving Mr Lock.
Mr Lock, who was 79, was killed by Matthew Daley following a collision between their two cars.
Daley admitted stabbing Mr Lock to death, claiming diminished responsibility, and was convicted of manslaughter in May.
Lewes Crown Court heard Daley suffered from chronic mental health problems and that his family had pleaded with the NHS to have him sectioned.
Sussex Partnership has admitted it "got things wrong" and should have carried out a formal assessment for Daley, who had a diagnosis of Asperger's - but who was also suffering from symptoms of psychosis.
Mr Lock's family has said they believe the NHS trust is to blame, saying he would "still be here today" if it had done its job properly.
An internal Sussex Partnership report into the care provided to Daley has not been published by the trust. An independent review of the case by NHS England is not expected to be published until next year.
In the new review, investigators found that in seven of the killings, there was criticism of how the the NHS 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".
In some cases, "risks assessments were not completed or were completed incorrectly" and "risk management plans were not completed".
The review said "some diagnoses are incorrect and remained unchanged in the face of the service user's behaviour".
Investigators 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. Policies also tended to view mental health service users as victims as opposed to potential abusers.
Colm Donaghy, chief executive of Sussex Partnership, offered his "sincere apology and condolences" to families.
He added: "We commissioned this review with NHS England because we want to make sure we have done everything possible in response to these tragic incidents.
"We have a responsibility to the patients, families and local communities we serve to ensure this.
"We have investigated each of the incidents individually. We also wanted independent, expert advice about any common themes which may link them.
"Sometimes, as is the case across the NHS, we need to improve processes, policies and training in response to incidents involving our services. But that isn't enough on its own.
"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.
"This goes beyond action plans; it's about organisational culture, values and leadership."
Marjorie Wallace, chief executive of the mental health charity Sane, said: "We are pleased that these steps are being taken to deal with the families who have been so often disregarded and who experienced obstacles in finding out the truth.
"We hope that they will be more included in future, but are concerned that even now at least some families have not been involved in this review."