Robert R. Reilly explains the irrationality of ISIS. He says there were two schools of thought at one time, and the irrational one won out.

Islamists do not live in what we might call historical time. Recall that for them the Qur’an is an ahistorical document. It exists in eternity. Also keep in mind that Ash’arite metaphysics guts historical time of its narrative meaning: time is a succession of unrelated events. ISIS adherents live in sacred time, which is static. In sacred time, everything is present all at once. This is why Islamists refer to Westerners in their literature as “Romans,” which is what seventh-century Muslim warriors called their Byzantine opponents. They are not being quaint. The past is present to them; that is why they must smash it if it does not conform to their beliefs.Ahistory fights history. This is why the Coptic Christians were faced north across the Mediterranean toward Rome when their throats were cut, as a warning that ISIS would next conquer Rome as Muslims once took Constantinople.