Blackmail fail
I would recommend against having dealings with the obviously mentally ill.  I'm not sure in what sense you might be legally liable (for a civil suit) if they happened to commit suicide a week later and mentioned you in the note.  Also, psychiatrists may be in many cases working off falsified doctrines just like any other doctor, but also like any other doctor, they really do know important things layfolk don't.  Leave the clearly crazy to the professionals.Though if this is your first real crazy caller I could understand where the fascination came from.  About... 15 years ago...? I got that email from someone who insisted that all I really needed to know was the Periodic Table of the Elements in which all answers were contained.  It took me a while to get that this was not someone I could save by argument.