This is the big picture summary statement that DU-folk desperately want/need to avoid, evade, and obscure:
But the religion preached by its most ardent followers derives from coherent and even learned interpretations of Islam.
ISIS, like A-Q, and their ilk are well within the mainstream of Islam. Adjectives like "fundamentalist" (utterly incorrect usage, but it's DU) or "extreme" (I've occasionally used the term for the sake of convenience) are forms of denial: ISIS, et al, are simply faithful to the Qur'an.