I'm trying to empathise with them... I get why people become radicals of other kinds... but saying no music, no sex, live by the book, behead those who disagree... what part of that makes people think "I want to get into this, this sounds fun"?
The Arab world has valid grievances, which motivated 9-11 for example, but there's more to it than that.
Step 1 is usually the West fucking a place up, while justifying it with Western moral sensibilities (cynically, they don't actually care and are there to steal resources/get cheap labor, eg British politicians/generals opposing eg women's suffrage at home while banning hijabs etc)
Violent resistance to that or Western intelligence financing radicals often follows (either organic because of the scale of the violence, or occupiers purposefully seeking to prolong the security crisis/justify further "investment" of military resources).