There is a somewhat long, but fascinating article in The Atlantic by Graeme Wood, which explains something that the mainstream media has gotten totally wrong.
What is ISIS (or ISIL, or Islamic State, or whatever the press is calling it this week)? What motivates them? Why are they beheading fellow Muslims? Why are some Muslims flocking to join them and other Muslims condemning them? Or as the article puts it:
The Islamic State is no mere collection of psychopaths. It is a religious group with carefully considered beliefs, among them that it is a key agent of the coming apocalypse. Here’s what that means for its strategy—and for how to stop it.
If you have any interest in understanding what is going on in the Middle East, I urge you to read it. After all, we have no chance of defeating them unless we have at least a passing understanding of them and their motivations.