Personally, I think that mass atrocities and beheadings is ISIS’ way of negotiating with the Americans over the issue of recognition of their de facto state. Because without recognition, even if unofficial, the state that ISIS is creating means little. With unofficial recognition, ISIS can make billions rather than millions of dollars from the sales of oil under their control, even if they have to sell it on the down-and-low. Recognition also allows ISIS the time it needs to consolidate its hold on the territories currently under its control, and to govern.
For yes, ISIS wants to govern, irrespective of the wishes of the population under its control and what the “experts” are saying about its inability to run institutions. These “experts” seem unable to see beyond the savagery of ISIS fighters and continue to reduce ISIS’ appeal to those willing to join its fighting units. In reality, ISIS has a sophisticated, if medievalistic, ideology, and its ranks are quietly swelling with people with the technocratic skills necessary for running institutions. These are the people who will actually be in charge of the ISIS down the road. But none of this can come true without recognition. ISIS’ leaders seem to think that Obama is a man who prefers to avoid confrontation at all cost, because Obama gave them plenty of reasons to encourage them to think along these lines. By beheading American and Western journalists and creating a crisis, of sorts, ISIS is reducing the confrontation to the fate of its Western prisoners. Soon, some of the people American officials meet with regularly in the region will offer to mediate, and talks will begin. The idea will be to reach a tacit last minute agreement, wherein ISIS begins releasing some of its Western prisoners, in exchange for stopping the airstrike and backing off. This will; quite similar to what happened back in August 2013 when Russia convinced Assad to surrender his chemical weapons arsenal to avoid American strikes. What worked for the goose could work for the gander. This might be what the Caliph, Abu Bakr Al-Baghdadi, is thinking.
Meanwhile ISIS will continue to expand beyond Syria and Iraq, with its representatives in Maan, Jordan, now declaring the city part of the Caliphate, and its units near the Lebanese borders busy punching carving out a base that can be used for future operations in Lebanon itself.
Feeling that this is what ISIS might be doing, Al-Nusra seems to be already following its lead by taking the UN workers in the Golan hostage. Assad, ISIS, Al-Nusra, the FSA and the Syrian National Coalition, they all want recognition and legitimacy. This is the currency all are after at this stage.