The case has caused an outcry because Kay, 28, was serving eight years in Maidstone prison for an almost identical attack three years earlier. In that shop raid - one of a string of robberies - his 17 year-old-victim, Richard Boston, survived after emergency surgery for a severed artery but has been left with a paralysed arm.
Kay, in attempting to rob Woolworth's in Teddington, south-west London, plunged a four-and-a-half inch blade into Mr Penfold's heart. In a police interview he said that any "have-a-go hero" deserved what he gets. He had no regrets.
He said Mr Penfold, 21, "lunged at me to stop me taking the money from the till. I stabbed him in the chest. I knew the blade had gone right in up to the handle. I did what I did because he went for me. That is what I had to do - that is all there is to it really."
The jury rejected Kay's defence that he was so mentally disturbed that he should instead be convicted of manslaughter. Prosecution and defence psychiatrists agreed he has a personality disorder and posed a great and continuing danger to the public. He will now go to Broadmoor top security hospital to be assessed to see if he is suitable for treatment.
French-born Kay first exhibited disturbed behaviour when he was 10. He was educated at a special school for maladjusted pupils, began offending at 16 and has a string of convictions for robberies at knife-point.
In December 1991, he was jailed at the Old Bailey for 16 shop raids and the near fatal attack on Mr Boston. In January last year, he was first allowed out of Maidstone on home leave. Within two hours and armed with a knife, he had grabbed pounds 2,000 from a post office - and then returned to jail.
On 25 August, he was allowed out for a further four days - and absconded. Six days later he snatched pounds 40 from a till in Woolworth's in Hounslow, south-west London, beginning a new sequence of nine robberies which were to culminate in Mr Penfold's death 10 weeks later.
Last night, the Prison Service said it had tightened up home leave procedures. It offered its sympathies to Mr Penfold's family. At the time Michael Howard, the Home Secretary, was reviewing home leave for violent prisoners, but it was not until two weeks after Kay's arrest he ordered a clampdown.
Four weeks before Mr Penfold's murder, a 33-year-old man serving eight years for the attempted murder of a girlfriend, was refused parole but had been allowed out on home leave from Maidstone. Three days later he was charged with the killing of his partner. He cannot be named for legal reasons.
The Maidstone cases were the latest in a string of incidents. Now prisoners with a history of violence would be unlikely to qualify for early release as key criteria include "public acceptability" and "public safety".Reuse content