Nelda Edwards died at her Hobart home in March 2016, two days after her husband of almost seven decades, 90-year-old David Edwards.
One of the couple's four sons, Stephen, a general practitioner at the time, was charged about one month later with murdering her.
The charge was dropped in 2020 because Stephen Edwards was ill, with Tasmania's Director of Public Prosecutions concluding there was no public interest in matters continuing.
In inquest findings published on Friday, coroner Simon Cooper ruled Mrs Edwards died as a result of lethal doses of midazolam and morphine administered by Stephen.
"(The) administration of those drugs ... was done by (Stephen) with the express intention of causing his mother's death," Mr Cooper said.
He concluded Stephen Edwards was lying when he told the inquest he administered a "cocktail" of drugs to help his mother sleep and for her back pain.
Mr Cooper said the drugs, especially morphine, were not used for those purposes.
"I have no hesitation in concluding he was lying - not mistaken or confused, but deliberately lying - during his evidence at the inquest," Mr Cooper said.
"(Stephen) claimed that an emergency of some type was the excuse for administering drugs prescribed for his father to his mother - there was no emergency."
On February 26, 2016, six days after the death of one of the Edwards' sons Glendon in Thailand, Mrs Edwards told a GP she wanted to die by an injection.
One of the couple's sons, Leigh Edwards, said the pair had stopped eating and drinking after Glendon's death.
He said he "absolutely" stood by Stephen's actions, saying his brother had provided appropriate palliative care.
Leigh Edwards (pictured) backed his brother, saying he had provided appropriate palliative care. (Ethan James/AAP PHOTOS)
"Some people may disagree (but) he was very well qualified. He had been involved in palliative care all his working life," Leigh Edwards said outside court on Friday.
"He was a doctor. He knew my parents better than anyone else."
Mr Cooper said Mrs Edwards had no need for palliative care and even if she did, there was no circumstance in which her son should have provided it.
The coroner noted she had several co-morbidities, including hypertensive cardiovascular disease, but they weren't responsible for her death.
David Edwards was ruled to have died of natural causes, namely lymphoma.
Stephen Edwards and brother Robert had charges of conspiracy, for allegedly providing misleading or false information to police, against them dropped.
The coroner said Stephen Edwards initially didn't tell police he had treated his mother or given her medication and denied knowing the cause of her death.
He told police she had indicated she wished to be with his father and said he held her hand when she died.
In a second interview with police, he gave a "materially different" account, admitting to administering significant doses of morphine, midazolam and clonazepam.
Leigh Edwards said his parents were a loving couple who didn't want to go to hospital and wanted to die in their home.
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