A convicted murderer will spend the next 40 years in prison after fatally stabbing a Good Samaritan who rushed to help a wealthy neighbour from being robbed.

Ian McLoughlin, 55, was sentenced at the Old Bailey for homicide for the third time after plunging a knife into the neck of Graham Buck as he responded to cries for help from his elderly neighbour, a convicted paedophile who had served time in prison with the killer.

Mr Buck, a father-of-three, managed to stagger to his home two doors away despite the gaping wound but died on the front lawn with his dog at his side, the court heard. In police interview, McLoughlin told officers: "I'm not sorry for what I did to the nonce, but I'm sorry for what I did to the pensioner."

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Mr Buck’s relatives were in court today as McLoughlin – who killed the 66-year-old on his first day release after serving 21 years in prison – admitted murder and was jailed for life. Mr Buck’s wife Karen mouthed “yes” as Mr Justice Sweeney said he would serve a minimum of 40 years behind bars.

In a statement to the court, she said that her life had been turned upside down by her husband's death. “There was no need kill him. He wasn’t a big man, if he got in the way he could just have been pushed over or even knocked out,” according to the statement.

She said killing him was the most “senseless, vicious” act of violence possible that had devastated his family. “We will be left with that thought and of his pain and suffering for the rest of our lives,” according to her statement.

McLoughlin was let out for a day from Spring Hill Prison for the first time in 21 years when he went to Francis Cory-Wright’s home in Little Gaddesden, Herts, determined to rob him. McLoughlin demanded “gold and silver” from Mr Cory-Wright, released earlier this year after serving time for abusing a 10-year-old boy, then tied him up as he filled a pillow case with heirlooms.

As McLoughlin dragged his loot downstairs, Mr Cory-Wright managed to struggle free and called for help from a window. Mr Buck, 66, heard his cries for help and went to his aid, but was dragged inside and stabbed with a dagger leaving a hole the size of a fist, the Old Bailey was told.

McLoughlin had killed twice before - the first time in 1984 when he hit Len Delgatty, 49, over the head with a hammer several times after a row, and left his body in a cupboard. He was jailed for 10 years for manslaughter at the Old Bailey, reduced to eight years on appeal.

After his release, he went to live in Brighton, where barman Peter Halls, 55, offered him work. McLoughlin said he assumed Mr Halls was gay, and thought that he may be expected to sleep with him. He stabbed Mr Halls to death after he claimed that he confessed to liking young boys and took holidays in Morocco for under-age sex, the court heard. In 1992 he was jailed for 25 years for murder.