Donald Mawditt sedated his wife Maureen before suffocating her with a carrier bag at their home. He then rang 999 and said: "I've helped my wife kill herself."
Mr Mawditt, a retired nurse, yesterday pleaded guilty to manslaughter on the grounds of diminished responsibility but walked free after a judge ruled that a jail sentence was not appropriate.
The grandfather, who had been married to his wife for 50 years, broke down in tears as he was given a three year conditional discharge.
Euthanasia campaigners said the case highlighted the need for a change in the law to protect those involved in mercy killings.
Bristol Crown Court heard that 70-year-old Mrs Mawditt had been diagnosed last April with haemo- cromatosis, whichleads to heart and liver failure and had left her in constant pain.
By December, her condition had deteriorated and she was told she only had a 50 per cent chance of living longer than two years.
She became incontinent, had difficulty speaking and suffered muscle spasms.
Mr Mawditt cared for his wife around the clock at their home in Weston-super-Mare in Somerset.
The court heard that the couple had made a pact some years ago that they would end the other's life if they became terminally ill.
Just after 11pm on 14 January this year, Mr Mawditt called the emergency services and said he had sedated his wife with Diazepam tablets before suffocating her with a plastic bag.
As he was led from the house after being arrested, he bent over his wife's body and kissed her, saying: "Goodbye, my darling."
Neil Ford QC, for the defence, said: "This was the ultimate act of love, compassion and mercy. It was a crime without wickedness."
Sentencing him, Judge Thomas Crowther said: "I cannot believe that any person who has heard what I have heard would regard a term of imprisonment as appropriate.
"You will never be punished for this act, save in so far that your suffering has been a punishment in itself."
Outside the court, Mr Mawditt would only say: "It's a part of my life that's over. I'm just shutting the door on the past."
His daughter Karen, 45, said: "I admire my dad for what he did. He definitely did the right thing.
"I know it's what mum wanted. I think she would have been proud of him too."
It is not known how many mercy killings take place each year, but campaigners say it could run into hundreds.
Deborah Annetts, chief executive of the Voluntary Euthanasia Society, said mercy killings would continue until the law was changed to give the terminally ill the right to die.
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