A man who axed a pensioner to death regularly visited the Old Bailey to watch murder trials, a court has heard.
Brian Messitt was hit on the head five times
Joseph Sheehan, 37, from Camden, became convinced Brian Messitt, his eventual victim, was trying to kill him.
Mr Messitt, 67, of Swiss Cottage, north London, died after he was hit over the head five times with an axe in March.
An Old Bailey judge told Sheehan - who earlier admitted manslaughter - he would be held indefinitely under the Mental Health Act.
Sheehan, who suffers from paranoid schizophrenia, has been remanded back to Broadmoor high security hospital.
After his arrest, psychiatrists thought he had been researching possible mental defences for himself during his court visits, but he later said he had been collecting evidence against Mr Messitt.
The psychiatrist concluded Sheehan was suffering from an abnormality of mind when he hit Mr Messitt. Judge Peter Beaumont described him as highly dangerous.
The court heard Mr Messitt, described as a loner and a cat lover, was the victim of a completely unprovoked attack in broad daylight.
"One witness saw Sheehan creep up behind Mr Messitt in a manner like actors in a pantomime," said Richard Whittam, prosecuting.