Apparently I think what goes wrong with religious fanatics is the same thing I think goes wrong with everyone else: they are liars.
The fanatic has to lie to himself, or he would discard his religion. That's the inherent nature of false religions. Accordingly, they will lie to you in a heartbeat. They're supremely untrustworthy.
It would seem fanaticism per se is a crime. I suppose should someone develop a true religion this wouldn't be a problem. On the other hand, fanaticism may be inherently incompatible with a true religion. It would inhuman after all. Alien. The potential fanatic would be forced to deal with it intellectually rather than emotionally. He would find it incompatible with his corrupt needs.