A paranoid cocaine addict killed his grandmother in a "frenzied and brutal attack", the Old Bailey has heard.
James Langlands, 26, unemployed, is on trial accused of killing Doris Langlands, 83, at her home in Winchmore Hill, north London.
He was arrested in Peterborough after the grandmother's body was found at the house on 4 April.
Mr Langlands denies murder, admitting manslaughter through diminished responsibility.
Earlier on the day of the killing he had spent time with his other grandmother, Millie Walton, the court heard.
After he was arrested, Mr Langlands suggested taking cocaine had brought on a paranoid psychosis, the jury was told.
Andrew Edis QC, prosecuting, said Mr Langlands claimed that his other grandmother's body language "conveyed the knowledge that she wanted him to kill Doris".
Mrs Walton told the court she and her grandson had watched football together and had a "normal" conversation.
Mr Edis said he believed that the killer acted rationally both before and after he struck.
Mr Langlands lived in Florida, in America, for five years before returning to the UK in 2008 after assaulting his wife, the court heard.
Mrs Walton said her grandson had been living with Mrs Langlands, who was known as Dot, but then went on to live in the family caravan at Tallington Waters, in Lincolnshire.
The trial continues.