It seems that IS is even in violation of Islam with the murder of Lt al-Kasasba.
There is a
hadith spoken by Muhammed himself that goes:
Originally Posted by Muhammad
Regarding The Abhorrance Of Burning The Enemy With Fire: (...) It is not proper to punish with fire except the Lord of fire.
There is even a fatwa out already. Sheikh Ahmed al-Tajib has decreed that this deed demands the most severe punishment under Sharia law: Crucifixion, other forms of the death penalty, and the amputation of limbs.
It may be that this terrible murder got something like a self-cleansing system within Islam to slowly grind into motion...