Tiernan Darnton, 21, was said to have “confessed” to the pair weeks after the funeral of Mary Gregory, 94, who died following a fire at her bungalow in Heysham, Lancashire, in May 2018.
The Lancaster & Morecambe College student spoke of setting the property alight and disabling a fire alarm before he left the address, Preston Crown was told.
Giving evidence on Tuesday, Alix Cozens said she asked Darnton about his “darkest secret” as part of the game.
She said: “He told us that he had killed somebody. I just questioned what he meant by it.
“He said he went to his grandma’s… and dropped a cigarette in the front room near some curtains or clothes, he had gone out and disabled the fire alarm and then had gone home.
“He said he did it because he didn’t want her to suffer any more.”
Prosecutor David McLachlan QC asked her: “Did you believe him?”
She replied: “Not at first because of watching horror films and CSI films, I thought ‘you are bluffing’.”
Ms Cozens said Darnton told them that following the fire he returned home, got changed and went back to bed.
Her friend, Liz Collinge, said before revealing his secret Darnton said he did not want them to “think any different about him”.
The witness said: “We thought that it was a joke.”
The court has heard that the death of Mrs Gregory, who had dementia, was assumed to be an accident until remarks made by Darnton in a counselling session in May 2019.
In the presence of his stepfather, Chris Gregory – the son of Mrs Gregory – the defendant said he set fire to curtains at the bungalow in Levens Drive in the early hours of May 28. Mrs Gregory died days later at the Royal Lancaster Infirmary from the effects of smoke inhalation.
Darnton was arrested at his family home in Combermere Road, Heysham, and his mobile phone and computer were seized.
Examination of the devices revealed a number of Google searches including “I want to cause evil”, “I’m a murderer”, “I’m a monster and I’m going to hell” and “urge to kill again”.
The prosecution says that far from being a tragic accident, Mrs Gregory’s death was caused by Darnton deliberately setting fire to her house.
Darnton denies murder.