A grandson calmly wished his grandmother merry Christmas and then viciously stabbed her to death as she lay in bed, a court heard.

Maxwell Twyman left the house to deliver presents and confessed to shocked family members: "I have killed my grandmother."

The 25-year-old labourer explained the horrific Christmas Day killing of Valerie Twyman, 62, to police: "She has always been a burden. That's why I am depressed."

A judge ordered that Twyman be detained in top security Broadmoor Hospital in Berkshire without limit of time after hearing he suffered from paranoid schizophrenia or a delusional disorder.

Maidstone Crown Court heard that he had been living with Mrs Twyman since February last year after his parents divorced.

Alan Kent, prosecuting, said Twyman's father Calvin tried to telephone his mother on Christmas morning but the call went to answerphone.

He called at the house in Conyngham Close, Ramsgate, but there was no answer. The curtains were drawn and he assumed she was still in bed.

Twyman, meanwhile, arrived at the home of his aunt and uncle Michelle and David Lloyd appearing, happy, normal and relaxed.

He wished them a happy Christmas and then said he had a confession to make. He bluntly told them: "I have killed my grandmother."

They at first thought he was joking. He told his mother not to go to Mrs Twyman's house, because he did not want her to see what he had done.

When asked what he had done, he replied: "Don't make a big thing of it."

He went on to claim that his grandmother had put faeces in his mouth when he was drunk at a party in 2003.

Family members went to the grandmother's home and Twyman's father broke in. They found Mrs Twyman lying on the bedroom floor. She had been stabbed 15 times.

Twyman spoke of killing other family members and a knife was taken from his shortly before his arrest.

Mr Kent said Twyman had smoked strong skunk cannabis since the age of 14, which made him feel lazy and unsociable. Doctors said his drug use was likely to have contributed to his mental state.

Twyman, escorted by five Broadmoor nurses, admitted manslaughter by reason of diminished responsibility.

Judge Michael Lawson, QC, said: "One can only shudder at the sadness they must all feel, not only for the loss of a grandmother, but the loss of the young man they knew.

"Violent death always causes distress. When it is the death of a beloved grandmother, caused by one of her grandsons it is difficult to imagine the extent of the sadness, anger and bewilderment of the family."

The judge added: "The court extends its sympathy, especially as the anniversary approaches."