Religion isn't actually a cause. It's a tool used by fundamentalists, and others. It's a means of controlling other people by appealing to their fears of death and solitude. There are many other tools, too. Ultimately, there are people who want to control other people, and they use those tools to do it.

In the case of ISIS, I have no idea what could motivate some young people, especially young women, to join their cause. Maybe some of the young men are eager to kill people, and see ISIS as means towards that end. Maybe they think they can make their fortunes. To me, though, what would be more interesting is to learn how many of them have become disillusioned since they got there, and would like to come back, but can't. That's the problem with such controlling groups: once you're in their clutches it can be damned near impossible to break free.