Tiernan Darnton, 20, made the revelation weeks after the funeral of Mary Gregory, who was thought to have been the victim of a tragic accident at her bungalow in Heysham, Lancashire, in May 2018.
He persuaded his two friends not to say anything about the disclosures, but nearly a year later he went on to make similar admissions to a counsellor who passed the matter on to the police, Preston Crown Court was told.
Dementia sufferer Mrs Gregory, a heavy smoker who liked to retain a degree of independence, was found in the conservatory area of the ablaze property in Levens Drive in the early hours of May 28 and was carried out by firefighters. She was treated by paramedics for smoke inhalation and then taken to the Royal Lancaster Infirmary where she died on June 1.
David McLachlan QC, opening the prosecution case, said after an initial investigation it was assumed the circumstances leading to her death were accidental until the remarks from the murder-accused in a counselling session on May 9 2019.
Darnton spoke of a friend who “could send me to prison coz of what he knows” and went on talk about an incident, said the prosecutor.
The female counsellor ended the session by saying to him “I’m not really clear what you’re saying but I think you’re trying to tell me you’ve killed someone”, to which Darnton nodded his head.
A week later, another session took place where the defendant was joined by his stepfather Chris Gregory, the son of Mrs Gregory.
Mr McLachlan said Darnton mentioned about committing a crime.
He said: “It was at this stage, the prosecution say, that Chris Gregory said ‘Is this all about my mum?’ and Tiernton Darnton volunteered ‘I set fire to the curtains with a lighter’.
“Chris Gregory responded ‘What, at 03:00 in the morning?’ and Tiernan Darnton replied ‘Yes’.”
A police investigation followed which led detectives to speak to Darnton’s friends, one who told of a Truth or Dare game they played weeks after the June 2018 funeral of Mrs Gregory.
Darnton was asked to reveal his “darkest secret”, and said: “I have a secret I haven’t told anyone. I may have killed someone”, the court heard.
When pressed, the prosecutor said, the defendant admitted he killed his “grandmother”, explained how he did it and said he did not want her to suffer any more as she had dementia.
Darnton was arrested in May 2019 at an address in Combermere Road, Heysham, and his mobile phone and computer was seized.
Examination of the devices revealed a number of Google searches in June 2018 including “murderer filled with despair”, “I’m a murderer” and “I’m a monster and I’m going to hell”.
Darnton made another search in August 2018 which read “feeling guilty for putting a loved one out of their misery”, the court heard.
Mr McLachlan told the jury that a fire examination expert would tell them the blaze was started by naked flame ignition near to the front bedroom window and not by dropping or carelessly disposing of a cigarette.
He said: “The prosecution’s case is that far from being a tragic accident, Mary Gregory’s death was brought about by Tiernan Darnton deliberately setting fire to her house.”
In a prepared statement to police, Darnton said that despite what he said to the counsellor he did not either deliberately or even accidentally start the fire.
He went on to say that his comments during the Truth or Dare game “had to be understood in the context of my mental health issues and in particular my attention seeking behaviour at that time”.
Darnton added he wanted to shock his “edgy” friends because he thought it would make them like him more.
He denies murder and an alternative count of manslaughter.
The three-week trial continues on Tuesday.