Former soldier Barry Rogers and his mother Penelope John have been jailed for at least 11 years each for the murder of his 84-year-old grandmother Betty Guy.
Former Army veteran Barry Rogers, 33, and his mother Penelope John, 50, plied the grandmother with whisky and pills before smothering her in bed.
The pair were found guilty at Swansea Crown Court of killing retired nurse Betty Guy following a three-week trial - after claiming she'd died from cancer in 2011.
But the court was told there were no record of Mrs Guy suffering from cancer, and Rogers was secretly recorded confessing to the murder.
It took the jury more than eight hours to find the pair guilty of murder.
They were arrested by police five years after Mrs Guy was found dead at her home in Haverfordwest, Pembrokeshire.
An ex girlfriend of former army lance corporal Rogers told police he had confessed to her about being responsible for his gran's death.
Police bugged John's home after the pair were bailed to the address, where Rogers was heard saying "But I did it" when his mum mentioned her murder arrest.
He was also recorded saying: "No, honestly, you have got nothing to worry about, it's me that's done the act.
"There's nothing to worry about. Nothing to worry about. Keep our story the same."
Prosecutor Paul Lewis QC said John plied her mother with a cocktail of diazepam, zoplicone and whisky, before Rogers smothered her to death.
He said: "They had together agreed to end the life of Mrs Guy at a time to be decided.
"On the late evening of Sunday November 6, 2011, Penelope John summoned her son to Mrs Guy's home to put their plan into operation.
"Penelope John gave her mother a cocktail of crushed tablets and alcohol, and Barry Rogers then deliberately killed her, probably by smothering her with an object such as a cushion or pillow."
Mr Lewis said Rogers confessed to ex Rhian Morris, the mother of his child, he'd given his grandmother "a load of tablets".
During their relationship he also told her: "You want to be careful or I'll do to you what I did to her."
Rogers later married another woman, Lisa Watkins, who said he'd told her how his gran had asked him to help her die.
He told the same story to high school sweetheart Sandra Adams, who reported his words to police after their relationship broke down.
Mr Lewis said: "He told Ms Adams that he had drunk a glass of whisky before putting a pillow over his nan's face.
"He said that his nan had been fighting him, so he stopped.
"He said he had then had another glass of whisky and had put a pillow over her face a second time.
"He said that this time 'she just went.'"
The court heard John rang 999 to say her mother had been suffering with bowel and stomach cancer and had died in bed.
But medical records showed Mrs Guy was never diagnosed with cancer, and a doctor John claimed had told her Mrs Guy was terminally ill said the conversation never took place.
No post mortem was carried out on Mrs Guy, and her daughter chose to have her body cremated days after her death.
John said her relationship with her mother had been "amazing" and had her ashes scattered in her garden which she visited every day.
She said she'd inherited only a washing machine and a tumble dryer after her mum's death, and her son's recorded admissions had been a "joke".
Asked about a mercy-killing pact, she said: "I would never agree to that. My mother was my life."
But Mr Lewis told the jury the pair had "repeatedly changed their stories", saying: "Our case is that they are both guilty of murder."