ISIS knows how to thrust its violence into the mainstream.
Given this context, ISIS’s insistence on an all-or-nothing caliphate isn’t “medieval” at all. It is a thoroughly modern group. It is executing a new and updated version of the early medieval Arab conquests. (In fact, good cases can be made for thinking of ISIS as shaped by Western political thought.)
As long as there have been conflicts among humans, there have been violent, often public, atrocities: the Roman conquest of Dacia (itself storyboarded on the famous Trajan’s Column in Rome), the Crusades, and the Holocaust. ISIS is part of a long tradition of demeaning one’s enemies through decapitation, incineration, disembowelment, and other reprehensible corporal punishments. But ISIS is better at it than almost any group that has come before it because its fighters are master propagandists, videographers, and photographers. They know how to thrust their violence into the mainstream. So, no, ISIS is not medieval. It is viciously modern.