Undated handout photo issued by Lancashire Police of Tiernan Darnton
A student has been sentenced to life in prison after murdering his own stepgrandmother by intentionally setting her curtain on fire.
Tiernan Darnton, 21, was sentenced to a minimum term of 15 years at Preston Crown Court on Friday.
Darnton killed Mary Gregory, 94 - his stepfather's mother - by starting a blaze at her bungalow in Heysham, Lancashire in May 2018.
When he was caught Darnton claimed it was a mercy killing but detectives found chilling internet searches on his phone including, 'good alibi' and 'I want to cause evil'.
The pensioner was discovered under a table in the conservatory and died four days after the blaze in hospital.
An inquest in 2018 ruled that Mrs Gregory was the victim of an accidental fire after Lancashire Fire and Rescue Service concluded the most probable cause was a dropped or carelessly discarded cigarette and ruled out any third party involvement.
But police reopened the case in 2019 after Darnton told a counsellor he used a lighter to set a curtain on fire at the address.
He claimed he started the blaze because he did not want Mrs Gregory 'to suffer any longer from dementia', Preston Crown Court heard.
However, police examination of Darnton's phone and laptop found internet searches made after the fire including 'I'm a murderer', 'I'm a monster and I'm going to hell', and 'Urge to kill again'.
A month before the blaze - when Darnton was 17 - he also searched 'Under 18 murder'.
Evidence shown to the jury included drawings found at Darnton's home address, outlining the floor plan of the 94-year-old's bungalow. They contained labels including 'good hiding place' and 'quick exit', as well as references to needing a 'good alibi'.
A packet of Tunnock's teacakes and a can of Coca-Cola, bought by Darnton from a nearby service station less than two hours before the fire, were also discovered at the scene.
During the inquiry it emerged the student had made a similar confession several weeks after Mrs Gregory's death during a game of Truth or Dare with friends, in which he revealed his 'darkest secret'.
He told them: 'I have a secret I haven't told anyone. I may have killed someone.'
The defendant closed his eyes briefly after the jury foreman returned the unanimous verdict and his stepfather rushed out of the courtroom as other members of the family broke down in tears.
Mary Gregory, who died following a fire at her bungalow in Heysham, Lancashire in May 2018
Evidence shown to the jury also included drawings found at Darnton's home address outlining the floorplan of Mrs Gregory's bungalow
Darnton, a former student at Kendal College and Lancaster & Morecambe College, was arrested at his family home in Combermere Road, Heysham, in May
