Shashank Joshi, of RUSI, has another article 'Where does the Islamic State's fetish with beheading people come from?':http://blogs.telegraph.co.uk/news/sh...eading-people/

He tries to answer this:
What, though, is the purpose of such brutality? The jihadists of the Islamic State (IS) are not, after all, nihilists. .....they are a highly professional military force, more similar to an army than insurgents, and seek a well-administered Islamic state. So why engage in beheadings and crucifixions?

First, psychological warfare is a key part of IS’s military strategy.

Second, IS understands that Western governments are, to some extent, dissuaded by the prospect of a British or American soldier meeting with a similar fate.

Third, terrorism is a form of propaganda by the deed. And the more chilling the deed, the more impactful the propaganda.
Now this is unexpected - well for me:
The first is that it can induce your enemies to fight even harder, because surrendering is such an awful option. One academic study shows that “the Wehrmacht’s policy of treating Soviet POWs brutally undercut German military effectiveness on the Eastern front”. Moreover, the Soviets’ own relative brutality to Germans meant that German soldiers fought harder in Russia than in Normandy. The lesson? IS can make its enemies flee, but it would be a foolish Iraqi unit that surrendered – and the net effect is that IS has to fight all the harder.