Heather Davidson, 53, from North Yorkshire, admitted that she smothered David Paterson, 81,(pictured above in the 1980s) in his care home but is expected to claim it was a mercy killing.
The woman met Mr Paterson, who was described as a loving family man with deeply-held religious beliefs, at church and had been visiting him at Sowerby House care home.
The pensioner was suffering with cancer and had said goodbye to some relatives and was expected to die naturally within days.
It emerged in court the woman had phoned the cancer charity Macmillan and told them about her intention to kill the elderly man.
In the dock Ms Davidson sobbed as she admitted murdering the pensioner.
She now faces a life sentence but the minimum term will not be handed down until June.
Under new legislation 'mercy killing' can be used as a defence, or mitigation, for the charge of murder.
But only certain people, mostly close family members, can use it.
In a statement after his death his family said: "David lived in the Thirsk area all of his life and previously worked as a welder at Thircon in Thirsk for 30 years.
"He was a devoted family man who adored his nieces and nephews.
"He was devoted to his wife and cared for her when she became ill, up until her death in March 2014.
"They were a very private couple who idolised their family and enjoyed being a part of their local community.
"David enjoyed writing poetry and had some of his poems published.
"He was a champion leek grower and also enjoyed quizzes and board games, anything that challenged his mind.
"He was an active member of his local church and enjoyed bowling until his health deteriorated and he was no longer fit enough.
"We will remember David as a devoted husband and family man who enjoyed the simple pleasures in life."