Part of the reason fanatics are so fervent is precisely because they don't believe. Their lack of faith is so obvious to them that they become paranoid. Surely you will notice? It's almost impossible not to notice... They try to flood the channel. Drown out the real signal with fake signals. The real signals keep coming at them, being real and all, which triggers them to flood the channel all the more vigorously...
Fanaticism is a cluster B personality disorder, possibly a proper subtype of narcissism. Like the narcissist, the fanatic is convinced their true self is unlovable and they must pretend to be someone else, or they will be cast into the fatal darkness beyond the fire.
What about the idea of accepting the evidence of their senses and the judgment of their reason? Totally beyond consideration.
Fanatics are obsessed with heretics because the heretic demonstrates their insanity. In fact regular folk tend not to mind heretics, or at least happily put up with them most of the time. This means the fanatic never had to oppress themselves so tyrannically. Better burn that fucker at the stake, rather than admit they screwed up. If reality conflicts with your insanity, so much the worse for reality.
Since fanatics reliably offer unjustified extreme violence, they are inherently criminal. Execution is likely a mercy in their case, and everyone else can only benefit from their removal. Being a fanatic is a very serious crime. All the stake-burnings were projection of what they accurately believe they themselves deserve.