A care home nurse told a court that she thought a male visitor was joking when he told her: "I just killed my nan."
Antony Jennings, 33, killed his 94-year-old grandmother Ruby Wilson by slitting her throat at the dementia unit of her care home in Buckhurst Hill in Essex on November 29 2017.
He has admitted her manslaughter on the grounds of diminished responsibility and denies her murder.
Giving evidence at Chelmsford Crown Court on Tuesday, where Jennings is standing trial, nurse Maria Stanciu described Mrs Wilson as a "happy person", adding: "She was sweet, she enjoyed singing."
She said Mrs Wilson (pictured) appeared happy to see her grandson and the pair drank tea together, spoke and laughed.
But later that day, Ms Stanciu said Jennings tapped her on the shoulder in a corridor and told her: "I'm sorry Maria, I just killed my nan."
"I asked him if he was joking and he said 'come'," she said.
Stephen Rose, prosecuting, asked: "In what sort of tone did he say these words to you?"
"He was looking at my face and he was smiling," Ms Stanciu replied.
She said she "went with him as I was thinking it was a joke".
Jennings, of Clinton Crescent, Ilford, east London, did not say anything during the one-minute walk to his grandmother's room, Ms Stanciu said.
She agreed with Mr Rose that she found Mrs Wilson in her armchair covered in blood and that she could not find a pulse.
"I ask him if he knows what he's done," Ms Stanciu said.
Asked if she could recall how Jennings replied, she said he told her: "She's in peace, she's not suffering any more, that she's not in pain any more."
She said she ran to her manager's office at Forest Place care home and police were called.
Cross-examined by Dorian Lovell-Pank QC, for Jennings, Ms Stanciu said Jennings was "so calm" and there was no smell of alcohol.
Asked by Mr Lovell-Pank what she meant when she told police that Jennings was like he was "on another planet", Ms Stanciu said: "His face was towards me but at the same time his mind was somewhere else."
The trial continues.
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