Well, they'll stone you when you walk all alone
They'll stone you when you are walkin' home
They'll stone you and then say they all are brave
They'll stone you when you're set down in your grave
--Rainy Day Woman, Bod Dylan
The Greek's Trojan horse is often considered to be the first example of special operations warfare, but the biblical story of David and Goliath predates the Trojan experience by several thousand years.
David was a simple shepherd who qualified annually with his sling. His projectile was a stone, and though he had no training equipment or experience at soldiering, he received a personal pre-ops briefing by Saul, his kingly lord and master. This was the beginning fo the Special Operations Forces concept that the man on the ground, regardless of rank, makes the tactical calls.
Fast-forwarding to current events unfolding in the newly forming Islamic State (IS) caliphate one can see the similarity to events played out since time immemorial. David's first act after killing Goliath was to cut off his head and triumphantly display his achievement to all comers. To witness was to verify and perhaps validate the killing.
IS is doing what men have been doing since honing cutting edges on tools. The head is a vulnerable wobbly basketball-sized thing on a thin stalk. "Off with her head!" is the ultimate action of a despot in power. Broadswords took some heaving, and the Guillotine was heralded as a humane form of killing, but the head has long been recognized as the thing to separate from the body to guarantee death.
Judith offing Holoferes head
Salome requested the head of John the Baptist, and act resulting in a Feast Day, and Judith beheaded Holofernes, showing the Israelites were fond of the practice, too. So to say IS is behaving medievally is incorrect; in fact, they are behaving Biblically.
We can say human behavior has changed somewhat since David's time. Now the Israelites must show restraint when dealing with attacks from Gaza. But if the situation were left up to Joshua of Jericho, Hamas, their followers and all bystanders, with the possible exceptions of breeding-age females, would be put to the sword. Literally.
Why the feigned horror today at behavior which is solidly within the monotheistic tradition? Beheading is a human behavior. Ugly, shocking, but nonetheless, human behavior.
The IS locals routinely behead their fellows; this is not unusual. Why not consider the possibility that the two recently decapitated American journalists were Central Intelligence Agency assets, and it was in fact this affiliation which led to their deaths. This is not to justify murder, but IS-type groups routinely view journalists as covers for participation in spying activities. Watching (spying) is what journalists do, even in their unaffiliated form.
What if David had been a nebbish and missed his target, or picked a more frangible projectile? Would we be worshiping a different God? Would our reactions be any more or less hypocritical?