Jeffrey Ash killed 83-year-old Ellen before setting fire to their home in Colston, Glasgow, leaving her remains inside the property.
A judge heard how the widow's condition had worsened and Ash was "driven to the edge".
It emerged Mrs Ash was released from hospital just a week before she died, despite her son admitting he was having difficulty coping with her care.
The 50-year-old fled to London after the death and handed himself into police where he said: "I killed my mum last night. I could not see her suffering any more."
Ash yesterday admitted culpable homicide and wilful fire-raising in a case Donald Findlay QC described as "desperately sad". The first-offender will learn his fate next month.
The High Court in Glasgow heard Ash was the primary carer for his mother at the home they shared. Mrs Ash's condition had deteriorated in the two years before she died, with her frequently confused and wandering from her house.
In March 2011, social work services considered 24-hour care for Mrs Ash but her son refused.
Prosecutor John Scullion said: "He declined stating that he was not ready to place his mother into a care home."
Mrs Ash, in her "lucid" times, had also begged her son not to put her in a home and he had agreed.
Mr Scullion told how the pensioner's GP confirmed that, in the period before her death, she was very difficult to treat and extremely unco-operative due to her illness.
The advocate depute added: "The accused continued to show appropriate concern for his mother, but indicated that he was having difficulty coping".
It reached the stage where Ash could apparently take no more.
Fire crews arrived at the home on March 21 to find Mrs Ash's burned corpse in the living room.
Her son told an officer at Belgravia police station, London, that afternoon: "I killed my mum last night. I smothered her then I burnt the house down.
"I could not see her suffering any more - she had Alzheimer's."
After being brought back to Scotland, he told police: "I am guilty and I want to do time."
Ash broke down in the dock as Mr Scullion narrated his confession.
Mr Scullion told how he spoke of his mother becoming worse and that he had begun questioning whether he could "end her life".
He said he later remembered causing her to fall to the floor before he covered her nose and mouth and "leaned with as much pressure as possible".
The court heard there was common ground between psychiatrists who examined Ash that he was suffering from an "abnormality of the mind" at the time.
Mr Findlay said: "Jeffrey Ash was a loving son, who lost his mother some years ago and was left to tend and care for someone he hardly knew."
The QC said it was patently clear Mrs Ash should not have been allowed home from the final time she was in hospital, but that her son coped as best he could.
Judge Lord Pentland deferred sentencing for reports.