devil of it
1. The worst or most unlucky thing about a trouble or accident; the part that is most regrettable.
Andy lost his notebook, and the devil of it was that the notebook contained all his homework for the coming week.
When I had a flat tire, the devil of it was that my spare tire was flat too.
2. Fun from doing mischief. — Used after "for".
The boys carried away Miss White's front gate just for the devil of it.