They saw how our citizenry went gah-gah over the relatively mild treatment of the prisoners at Abu Ghraib, and perceived us as weak. They know that the world will condemn us for that, while they go on torturing and murdering non-combatants.
The world community who are victims of the terrorist actions should renounce the Geneva Convention and put all terrorists on notice that they cannot rely on individual state/country's laws to provide them with humane treatment when they are captured.
That's the way to stop their atrocities in their tracks.
"Well, I think that the Geneva convention should not apply to treatment of terrorists in captivity."
It does not.