Lessons can be learned from a report into the treatment of murderer Michael Stone, mental health experts have said.
Lin and Megan Russell were killed 10 years ago
Stone, who is serving life, had asked for help before he attacked mother and daughter Lin and Megan Russell in Kent in 1996, the report found.
Responding to the findings, the Mental Health Alliance said Stone had been "lost between health, criminal justice and substance misuse services".
"People should not be turned away when they seek help," its spokesman said.
He added that it was a common problem and called for improved co-ordination between services.
"Better services, properly funded with more effort to intervene early and improve joint working around users' needs are key to reducing risk," he said.
"No legislation can ever prevent all tragedies from happening. We can learn lessons when they do, and respond appropriately."
Dr Marcus Roberts, head of policy for Mind, said more treatment services needed to be in place and accessible to people like Stone.
"What we do know about Michael Stone is that he did try to access services, but because of the level of pressure on drug treatment services in the Kent area at the time, he did not get the treatment he needed."
"We need the services to respond better because too often people are slipping through the net," he said.
"In the majority of cases the people who are in need of treatment come to harm themselves but sometimes, other people are harmed."
The Royal College of Psychiatrists said the report highlighted the need for improved funding of mental health services.
Honorary vice president Dr Tony Zigmond said: "The report emphasises the need for a properly-resourced multi-agency service for people with mental health problems and drug dependencies.
"Mental health services have long been under-resourced and the current reduction in funding will make matters worse."
The Mental Health Alliance said the government's amendment to the Mental Health act should give people the right to be assessed and treated when they request it - as now applies in Scotland.
The independent inquiry which found failings in Stone's care but said it was unable to say the deaths could have been avoided, was commissioned by the three agencies which had been treating or supervising him.
They were the West Kent Health Authority (now NHS South East Coast), Kent Social Services and Kent Probation Service.
They acknowledged that mistakes were made and they failed to share information, but also said a number of recommendations had already been acted on.