Have a read of the following article (http://gu.com/p/45tjc), it is a very interesting insight into the fundamentalist ideology of Isis. Here is an extract if you are not keen to read a whole article!
Savagery is at the core of Isis ideology. But it is crucial not to play down that brutal acts have to be justified through sharia texts. Islamic fundamentalism is Isis’s ideology, so to speak, and every act has to be grounded in religious traditions. Muslim clerics who issue a “letter to al-Baghdadi” or a lengthy fatwa to delegitimise Isis miss the mark unless they understand the invigorating nature of this violent ideology. While Isis uses manuals such as Naji’s book, it references religious texts and stories. Muslim clerics should recognise that theoretical fatwas cannot sufficiently counter what I call “kinetic” sharia, consisting of stories and actions carried out by authoritative Muslim figures in early Islam, on which Isis relies heavily to justify its ideology. Statements such as “this hadith is weak” or “it is not permissible to kill prisoners of war” can be backed by religious texts, but how early Muslim leaders acted is similarly powerful, if not more persuasive.