Grant that ISIS aren't really muslims. Grant that their motivations are not, at heart, religious at all - they are entirely secular. They are moved by desires for wealth, for housing, for jobs.
Alright. Carry that to its conclusion then - ISIS is a group of atheists, with secular motives. They're about as religious as a huckster televangelist saying God demanded donations. They do not kill for the glory of God - they want money, for the sake of comfort, and shelter, and food, and all the usual reasons.
I have a feeling that this will turn out to be wrong as well, but at that point ISIS becomes utterly mysterious, and we're left with the grim irony that their motivations cannot be attributed to natural forces.
But wait, that won't be accepted either...
It's amazing how complicated issues get when the most obvious and straightforward conclusions are declared impossible lest they offend regressive sensibilities.