Heather Davidson, 54, admitted using a pillow to murder David Paterson, 81, in his care home in February but she had argued it was a mercy killing.
Teesside Crown Court heard how Davidson, from Sowerby, Thirsk, had phoned Macmillan Cancer Care to speak about her friend, who she had met at St Mary's Church in Thirsk.
She said he was "skin and bone" and in a "dreadful state".
"It might be better if I could put a pillow over his head," she told the call handler.
"Would I be a murderer if I did that?," she asked, "In the eyes of the law, yes you would, yeah," he replied.
"If he was a dog he would have been put down months ago," she said.
Two hours later she had murdered him, despite, the court was told, his opposition to euthanasia.
Police were called by the Macmillan but the number could not be traced in time.
Mr Paterson was just days from a natural death and prosecutor Jonathan Sharp told the court he was "a devout Christian" with "strong ethical objections to euthanasia".
"He had said it will be God's decision, and only God's, when it was his time to meet his maker," the lawyer added.
David Paterson pictured in the 1980s
Judge Simon Bourne-Arton: "You were only were saving him a few hours of suffering.
"In so doing, you deprived him of what he wanted most, a natural death. This private man did not in death have a private ending."
He was the "pillar of his local church" and had lost his wife in March 2014.
Davidson met the pensioner and became friends with him at a church group for people with mental health issues, which he volunteered at.
In a statement, the family of David Paterson said: "She should have known he would have wanted it to be God's will when he died and allowed him to do so in privacy and with dignity - she had no right to do this to him.
"We were anticipating a funeral that would be a celebration of his life but, instead of having the space and time to remember all the good he brought, this has thrown our lives and thoughts into chaos.
"We all have moments considering the horror of Uncle David's last few minutes and console ourselves with the thought that, as he had been almost unconscious for most of the time, he would hopefully not have known anything about it.
"Our overriding emotions are of shock and horror. We all want to know why this has happened. It makes no sense at all.
"Heather Davidson was a church-goer, a person Uncle David had helped, a person he trusted and considered a friend. She has taken away our opportunity to grieve for him, as we had the right to do."
The Crown Prosecution Service said in a statement: "Mr Paterson had previously firmly expressed his opposition to any intervention in the dying process.
"As a fellow church-goer Davidson would have been well aware that her actions went against Mr Paterson's most profound beliefs. Equally, her actions went against his family's deepest wishes.
"Her decision to end his life deprived Mr Paterson of what he and his family most wanted - a natural, private and dignified death. Her actions also denied his close family of being able to say their final farewells and to grieve privately.
"I hope the sentence imposed today will enable Mr Paterson's family to begin to come to terms with the tragic circumstances of his death. I am sure they would want him to be remembered by how he lived his life, not for how it ended."
Press Association reporter Tom Wilkinson was in court and spoke to Premier's Antony Bushfield:
