An NHS trust failed a man with mental health problems before he murdered a retired solicitor on a road following a minor car crash, a court heard.
Relatives of Matthew Daley, 35, were "constantly on the case" of clinicians to try to get him the treatment he needed, it was said.
And at one point, his desperate father said if he was not given proper care, he would go on to "hurt someone or worse", a jury was told.
Lewes Crown Court heard Sussex Partnership NHS Foundation Trust has apologised to Daley's family for having "failed" him in his care and treatment.
Daley is accused of murdering 79-year-old Donald Lock by knifing him 39 times as he drove back from a cycling meeting on the A24 at Findon, near Worthing, West Sussex, on July 16 last year.
Jurors heard Daley (pictured) stabbed Mr Lock repeatedly after his Toyota crashed into the back of Daley's car at about 16mph, causing minor damage to both vehicles.
One witness said Daley, who is being held in a medium-secure unit, looked like he was "having a passport photo" taken during the attack, describing him as "expressionless".
Following the knifing, Daley then drove off and abandoned his Ford Fusion at Woodlands Stables, the court was told.
Opening the Crown's case, Philip Bennetts QC said: "On July 16 of last year this defendant Matthew Daley killed Donald Lock.
"Matthew Daley broke violently approaching the junction of the A24 in Findon, West Sussex. Donald Lock, who was immediately behind him, collided with his car.
"Donald Lock got out of his car and went to speak to Matthew Daley. Matthew Daley got out of his car as Donald Lock approached."
Mr Lock first asked Daley why he had stopped so suddenly before stabbing him repeatedly, telling the pensioner to "die you f***ing c***", jurors heard.
One witness said it was a "ferocious" attack, adding: "Every second it looked like he stabbed him twice."
Another said "calm" Mr Lock had tried to get Daley in a "bear hug" to try to stop getting hit.
Jurors heard Daley had performed a full emergency stop before the crash and got out of his car before "flailing punches" at Mr Lock.
In interview following his arrest, Daley said he wanted to protect himself, claiming that Mr Lock was "really furious and angry".
Daley's defence counsel David Howker QC said it was beyond argument that Daley suffered from mental health problems, adding that the jury would have to consider whether he acted with diminished responsibility.
Outlining Daley's mental health treatment, he said: "We will call evidence about Mr Daley's mental health difficulties over the past 10 years and how he came to be diagnosed by the NHS as suffering from autistic spectrum, and how it was that that diagnosis was not a thorough diagnosis of his condition.
"And we will call evidence of how Mr Daley's family, particularly his father, were constantly on the case of clinicians, even at one stage, in an act of desperation, Mr Daley saying, 'Look, if you don't give my son the proper care or treatment he needs he is going to hurt someone or worse'.
"And I'm afraid to say those fears were realised."
He went on: "The health authority belatedly have acknowledged to the Daley family how they failed Mr Daley, how he had not received the care and treatment he should have done and they have apologised for that.
"There will always be a question mark that if done properly, that this could have all been avoided."
Daley, formerly of St Elmo Road, Worthing, denies murder.
The trial, before judge Mr Justice Singh, is expected to last around two weeks.
In court to hear the case were relatives of Mr Lock, including his widow Maureen and son Andrew.
Witness Andrew Slater told how he tried to remonstrate with Daley, but he continued to carry out a "frenzied" attack on Mr Lock.
He said the main aspect about the knifing that he could recall was the silence, with no-one saying anything.
As Mr Lock was being attacked, Mr Slater told Daley: "'Come on mate, leave it out'.
"He stopped and that's when I saw he had a knife in his clenched fist. I went, 'Oh my God'.
"As soon as I saw the knife, I backed away slowly, fearing he would attack me, and I got back into my car.
"Daley looked both ways and then carried on stabbing Don in the side of his back several times.
"I was in my car completely shocked. I didn't know what to do and I was shouting 'Oh my God, oh my God'.
"After a frenzied attack, Daley looked again up and down and, almost in a crouched movement, went to his car and wheel-span away.
"I went to Don and shouted, 'Can you hear me? Can you hear me?"
Mr Lock, who had recently been given the all-clear from prostate cancer, died at the scene.
Another motorist, Abby Smith, fought back tears as she recalled seeing Mr Lock being stabbed, and how moments earlier she had to do an emergency stop after Daley brought his car to a sudden halt in front of Mr Lock's vehicle.
Miss Smith told the court how the pair became engaged in a bear hug before she saw Daley rain punches on Mr Lock.
She said: "Mr Lock didn't really react at all, he didn't fight back, nothing. My younger sister was about to get out to assist Mr Lock.
"It was then that I noticed that Mr Daley had a blade and I grabbed my sister back into the car. I was frightened."
The case continues from 10am tomorrow.
Copyright (c) Press Association Ltd. 2016, All Rights Reserved. Picture (c) Gareth Fuller / PA Wire.