My angle...
They are at war with civilization, therefore, we’re at war with them. We’re willing to abide by the Geneva Convention, as a contract between parties at war, so long as it is honored by the enemy. If it is not honored the contract is breached, it is null and void. Therefore, we should not honor it with respect to ISIL. Kill how they kill. For every murder they commit we return in kind with one of the Gitmo prisoners of war.
As they obviously think that these forms of brutality are frightening, they need a taste of their own medicine. So long as it’s only their side that is doing this they’ll continue. The second they think that it might happen to them, if caught, it’ll stop.
They are trying to frighten the political leaders who command air forces. No leader wants to be in charge when one of his country's airman is executed in such a brutal manner. Isis is hoping the leaders will be intimidated and not order their forces against them.