If you sincerely believe yourself to be God and you are not, you are insane, aka - not in touch with reality.
It can be argued hun calling himself a prophet and the son of man isn't necessarily claiming to be God. You should also know I don't believe NT texts to be inspired. I believe the guy was an observant Jew and later writers, redactors and interpolaters made him into the man-god figure. So its not him that's necessarily the madman. Its the pointy hats that came after him.