If you listen to what they say ISIL nearly always provides a reason for their action and supply their justification.
They just don't murder, rape and steal in a theological vacuum.
To the Christian ear we may be tone death to their message and certainly the secular media refuses to even discuss the matter, but they do provide jurisprudence for their actions.
Surat Al-'Anfāl (The Spoils of War)
Sahih International 8:12
[Remember] when your Lord inspired to the angels, "I am with you, so strengthen those who have believed. I will cast terror into the hearts of those who disbelieved, so strike [them] upon the necks and strike from them every fingertip."
