A former soldier accused of smothering his grandmother told police he would not have killed her or anyone else because he "shot someone in Iraq" and "that is why I am messed up in the head now", a court has heard.
Barry Rogers, on trial along with his mother Penelope John for the murder of Betty Guy, told officers that he was the 84-year-old's "blue-eyed boy", during an interview after he was arrested in 2016.
In a video of the exchange played at Swansea Crown Court on Friday he said he could not have killed the former nurse because she was already dead by the time he arrived at her home in Johnston, Pembrokeshire, on November 7 2011.
"I shot someone in Iraq," he said.
"I would not kill anyone else, that is why I am messed up in the head - because of Iraq."
Rogers, 33, said his mother called him just before Mrs Guy died but he did not remember what she said.
He then drove to his grandmother's home from Frome, Somerset, where he was living at the time, "doing 140 miles per hour on the motorway".
"I drove from Frome to Johnston pretty fast and by the time I got there my nan was already dead," he said.
"I knew for weeks, for months that my nan was ill.
"We all knew it was coming.
"I think my mum phoned me, I can't remember exactly what was said or the details, but she phoned and I should have left straight away but I didn't, I left later ... I felt guilty then because I should have left earlier and if I left earlier then I might have seen her."
The court previously heard Rogers, who said he suffers from PTSD and borderline personality disorder, told three girlfriends that he had helped his nan "pass", in the years that followed her death.
He and John, 50, who is accused of feeding her mother a cocktail of pills and whiskey, came under suspicion after one of those women, Sandra Adams, reported what he had said to police.
Rogers first said that she was "vindictive".
He claimed the three women must have colluded but later said it was possible he had said those things but he did not recall it due to his mental health problems.
"I get very fiery," he said.
Rogers said did not see Mrs Guy enough in the months leading up to her death but he tried to help out with odd jobs.
Rogers, of Fishguard, Pembrokeshire, who was crying during parts of the interview, said he was "gobsmacked" when officers came to arrest him and agreed that he had said: "I just want to know who grassed me up."
He told the interviewers Mrs Guy was "old and frail" and while he did not know the specifics of her medical conditions she had "complications" and used to keep a "big bag of tablets" next to her.
The court heard John, of St Dogmaels, Pembrokeshire, called the emergency services to Mrs Guy's home on November 7 2011, saying she thought her mother had died and claiming Mrs Guy had been suffering from stomach and bowel cancer.
Police installed covert recording equipment in John's home while they were in custody being interviewed in 2016.
In one exchange, Rogers said to John: "No honestly, you have got nothing to worry about, it's me that's the one that's done the act."
Jurors also watched footage of a later interview with Rogers when he was asked about the recordings.
He said he knew there was a bug in the home and that he had made the comments "to take the p*** out of you pigs".
"I was expecting a bug to be placed in the property so I purchased a bug device," he added.
"When that bug device confirmed that there was a bug in the property my suspicions were obviously right."
Rogers and John both deny murder and the alternative charge of manslaughter.
The trial continues.