Sha'irullah
رسول الآلهة
I am of strong Jewish stock, my father was a lawyer and my mother an economist.
. . . I am being sarcastic
This is fickle because when Daesh (ISIS's real name) started they were very sloppy and Islamically did not even perform execution right. Yes they are permitted to kill in such a gruesome manner but there are cases of them executing people who are either way to young or are technically speaking not a kafir.
You need to remember that in the hadith there was no divide between Muslims so Muslims did not worry about if Hanafiyyah, Shiatu Ali', or Maliki'yyah was a legitimate way to practice Islam. Truly speaking they have no right to kill Jews for being strict monotheists. Yet they claim all sorts of individuals kafr without proper justification.
Sure there are grey areas as I am discussing but taken into account of this, Daesh should be killing Christians en masse and perhaps Shi'iya but instead they waste their time doing executions to legit Muslims canonically speaking.
But at the same time in Kitab Sahih al-Bukhari it is stated that the Muslims should fight till the populence behaves Muslim and prays to the Qiblah which is the direction of Makkah. Yet again Daesh is killing people who already do this granted the religiosity may not be very high.
BUT the direction of Daesh and their dishing out of punishments are entirely halal on the other hand. The cutting of hands for thievery and etc are as they say, "taqdim ila Allah." This means in obedience to Allah's will.
It is difficult because purposeful suicide is deemed an entry into Jahannam yet the notion of suicide bombing can be interpreted as an act of war or aggression. The context varies and the usage of martyrdom has always been an aspect of Islam for a millenia.
