A broadcaster for the BBC's Inside Out programme smothered his ailing lover to spare him from "terrible pain".
Police are investigating after the Nottingham film-maker said he had a pact with the deceased man to act if his suffering increased.
During a documentary on death and dying, Ray Gosling, 70, said: "I killed someone once. He'd been my lover and he got Aids.
"I picked up the pillow and smothered him until he was dead. No regrets."
Mr Gosling said he smothered his former lover while he was in a hospital - which he did not name - after doctors told him that there was nothing further that could be done for him.
"I said to the doctor: 'Leave me just for a bit,' and he went away.
"I picked up the pillow and smothered him until he was dead.
"The doctor came back and I said: 'He's gone.' Nothing more was ever said.
"When you love someone, it is difficult to see them suffer. My feelings on euthanasia are like jelly - they wobble about.
"This is the time to share a secret I have kept for quite a long time."
Nottinghamshire Police have started an inquiry.
A spokesman said: "We were not aware of Mr Gosling's comments until the BBC Inside Out programme was shown.
"We are now liaising with the BBC and will investigate the matter."
Dr Peter Saunders, of pressure group Care Not Killing, said the facts need to come out in court.
Dr Saunders said: "We have a case, by Ray's account, not of assisted suicide but of intentional killing or murder."
Assisted suicide remains a criminal offence, but interim guidelines issued in September by the director of public prosecutions set out the factors which weigh in favour of and against prosecution in different cases in England and Wales.
Last month Kay Gilderdale, 55, of Stonegate, East Sussex, was cleared of the attempted murder of her severely ill daughter who had ME. Mrs Gilderdale had administered lethal drugs to end 31-year-old Lynn Gilderdale's life after her daughter called her for help when her own attempts at suicide failed.
Days before that another mother, Frances Inglis, 57, of Dagenham, east London, was jailed for nine years for murder after she injected her brain-damaged son Thomas, 22, with a lethal dose of heroin.
'Let it be'
Mr Gosling, asked by Inside Out presenter Marie Ashby if he had regrets, said: "Absolutely none. He was in terrible pain - I was there and I saw it. It breaks you into pieces. I don't think it's a crime.
"If he was looking down on me now he would be proud that I did it and proud I've told other people."
When asked about the dead person's family he said: "Some know, some don't. It's best that way. Let it be."
Mr Gosling, a writer and broadcaster of hundreds of radio and TV documentaries, wrote a documentary about his decision to move into sheltered accommodation in the documentary Ray Gosling OAP.
The programme won the Jonathan Gili most entertaining documentary award at the Grierson 2007 awards.
• The interview with Ray Gosling was broadcast on BBC One's Inside Out East Midlands programme on Monday evening.