Considering how much success ISIS has enjoyed in the last year or so it surprises me how many people still seem to struggle with the concept that quite a few people in the occupied territories might be supportive, sympathetic, or even tolerant, of ISIS.
It is perfectly feasible that many people willingly collaborate with ISIS, and some even join them.
I can only speculate, but I imagine when you’re living somewhere that is basically a perpetual warzone any group that looks like “winning” and establishing some kind of stability would be preferable to open warfare. They’re not monsters or demons, just men. Some of whom do monstrous things.