Look how the army cut and run....they KNOW these beasts and will not fight them.
Unfortunately for the 'moderate' Muslims, the ISIS campaign is primarily to eliminate the weak sisters and lukewarm jihadis from the ranks. The idea is that the Caliphate will be the home to the most worthy (who survive, of course).
Christians and Jews who get caught up in the juggernaut are fair game to the hardcore as well.
All the medieval horrors they can bring forth only serve to strengthen the 'faithful' and keep them in line (as horror has been used forever)--PSYOPs against the foe, warnings to the faithful--7th century 'shock and awe'.