A High Court judge says a 27-year-old, on the balance of probabilities, killed a 54-year-old esteemed entomologist

Stephen Thorpe was fatally stabbed near the Blockhouse Bay Tennis Club in August.

At the High Court at Auckland yesterday, Justice Graham Lang said the defendant, who has name suppression, can't be found guilty because of his mental illness.

The court's heard the man's mental health history, including reported hallucinations of the devil and bouts of anger.

He's returned to the Mason Clinic - a lockdown psychiatric facility where he's been since his arrest.