A son who strangled his 81-year-old mother with her dressing gown cord told police she had begged him to end her suffering, the Old Bailey has heard.
Thomas Quinn, 50, claimed his mother Philimena told him she was suffering day in and day out.
But the prosecution say Mrs Quinn showed no signs of wanting to die and was still sociable despite being housebound.
"Is the truth that he snapped, lost his temper and strangled her - leaving her sprawled on her bed for the police to find?" Sarah Whitehouse, prosecuting, asked the jury.
Quinn denies murder. He has pleaded guilty to manslaughter but this is not accepted by the Crown.
Miss Whitehouse said heavy drinker Quinn had gone to live with his mother in her small flat in Islington, north London, after splitting up from his partner.
Mrs Quinn had suffered a stroke but was still able to walk with a stick and enjoyed visits to her local priest and a pastor.
The priest called police in June last year after being unable to get an answer from the flat for about a month.
Officers found Mrs Quinn's decomposing body with the cord still round her neck when they looked in the taped off bedroom.
Quinn had told the officers his mother died a couple of weeks before.
He said: "I did what she asked me to do to end her suffering day in and day out."
Miss Whitehouse said Quinn did not repeat the statement but the defence would call doctors to say he was depressed.
She added: "He may have been somewhat depressed stuck in a small flat caring for his mother but his problems were not so severe that he was unable to form a rational judgment."