Barbaric though Isis may be, what is less than well known is the barbarity of its ideology. Their theology is that of Egyptian Islamist Sayyid Qutib who believed the Muslim community to have been "extinct" in the absence of global leadership, because of which radical and violent change is justified. Like the al Qa'idah organisation in which they have their genesis the writings of Muhammad ibn al-Wahhab are important to Isis.
Ibn Taymiyah is a further reference point as the writings of Muhammad ibn al-Wahhab suggest. This theology was fine tuned by the late Youssef Al-Ayyeri, a young Saudi drift car enthusiast turned ideologue killed some years ago by Saudi security forces. Prior to his death he became leader of Al Qa'idah in the Arabian Peninsula. In a short booklet written just before he was killed he suggested that the concept of tolerance for other religions (as in the "People of the Book" idea) was from the early period of Islam when the religion was vulnerable. He claimed that this approach was not adhered to by the Prophet Mohamed in his later years. The world is thus divided into Dar al Islam (the House of Islam) and Dar al Harb (the House of War) and the simplistic puritanism of Dar al Islam is contrasted with the decadence of Dar al Harb.
Daish / Isis rejects democracy, rejects socialism, and rejects nationalism (nationalism stands in the way of the concept of an Islamic empire) in favour of universal rule by a dictator-Caliph brought about by war. Indeed a war that eliminates the middle ground and sets the stage for the final eschatological battle between good and evil is viewed as desirable.
The avowed intent of Isis is to conquer Mecca and Medina.