How does Islamic State justify its atrocities in name of Islam?

News article, posted 02.08.2015, from Washington DC, United States, in:
Countless Muslims have condemned the actions of the Islamic State, calling them extremists and not true followers of Islam. The Islamic State themselves have justified their actions by selectively picking specific Quranic verses out of context or simply say that they are waging a jihad for Islam and representing aggrieved Muslims. An interview with Islamic scholar Dr. Jocelyne Cesari addresses many of the issues revolving around the Islamic State including their reasons for jihad, justifications for brutality, and the decline of Islam in modern times.