Fanatics can cause a very great deal of harm, can't they? What is it that makes people become fanatics, anyway?

M-W:

Main Entry: fa·nat·ic
Pronunciation: f&-'na-tik
Variant(s): or fa·nat·i·cal /-ti-k&l/
Function: adjective
Etymology: Latin fanaticus inspired by a deity, frenzied, from fanum temple -- more at FEAST
Date: 1550
: marked by excessive enthusiasm and often intense uncritical devotion <they're fanatic about politics>

I suppose we're talking being possessed here, aren't we? The danger is the "uncritical devotion" part, which makes everything black-and-white, with no shades of grey and no compromise.

It has the huge benefit of simplicity and [thanks eta] freedom from doubt. Quite easily achieved, too - you just short out your faculties of reason, your concerns and troubles, and let your unequivocal passions guide you; although I suspect they only appear unequivocal because they get hitched to a ready-made creed.

It's terribly easy to mistake relief from discomfort and uncertainty as a sign you're going in the right direction.
As such fanaticism is appealing and highly infectious, even to its enemies.

Hey ho, I'm a jolly fella [musical notes]