First, I would say, is ideology. I have never spoken to any member of these groups, not just ISIS, but also Al Qaeda, Shabab, etc., who wasn't driven by the ideology. Beyond that, a lot of people are wounded in some way - they've fallen out of society in some manner. All of the ones I've spoken to seem to have veered off course from lives we consider acceptable and successful. Of course, there are others like Osama bin Laden, who was a wealthy and successful businessperson.
