On his return, he was diagnosed to be of unsound mind, and was taken by his family to recuperate in the countryside village of Cobham. In 1843, Dadd murdered his father with a knife whilst deluded, believing him to be the Devil in disguise, and fled for France; en route fo Paris Dadd attempted to murder another tourist with a razor, but was unsuccessful and was arrested by the police. Dadd confessed to the murder of his father and was returned to England.
He was committed to Bethlem psychiatric hospital, and was diagnosed as having what is now known as bipolar disorder. In hospital he was allowed to continue to paint and it was here that many of his masterpieces were created. He was later transferred to Broadmoor, another psychiatric hospital.
His (perhaps most) celebrated painting "The Fairy Feller's Master-Stroke" was to become the title of a song by rock band Queen.