After returning to England, he became involved in a millennialist movement and came to believe that the Second Coming of Jesus Christ would be advanced if he himself were killed by the British government.
Hadfield pleaded insanity but the standard of the day for a successful plea was that the defendant must be "lost to all sense ... incapable of forming a judgement upon the consequences of the act which he is about to do".
[1][2] Up to that time, defendants acquitted by reason of insanity had faced no certain fate and had often been released back to the safe-keeping of their families.
Hadfield was detained in Bethlem Royal Hospital for the rest of his life, save for a short period when he escaped.
He was recaptured at Dover attempting to flee to France and was briefly held at Newgate Prison before being transferred to the newly opened criminal department at Bethlem (or Bedlam, as it was known).