Personally, I really dislike the ones where they use child soldiers for their combat.
Imagine, from the moment you were born, being groomed by a group of terrorists to believe that everyone who doesn't share the same beliefs as you is an enemy that deserves to be slaughtered.
Imagine, then, that they teach you that death is desirable. That dying for their cause will send you to a grand, beautiful heaven. They teach children to disregard their lives, their futures, and go out and blow themselves up. To kill innocents for simply not praying to the same god that they do. That god will reward them eternally for it.
They lie to the children and use them as cannon fodder. Maybe I'm simply too emotional, but it makes my fucking skin crawl.