It's also their strategy in Syria, they basically either bought, or starved their FSA and Al-Nursa rivals in the north and are doing the same with the pockets of the SAA that still remain there
Again, Mind elaborating this claim a bit?
It's major sources of income and natural resources are located, ISIS wants to capture as much of these areas and starve the rest out, this is show by their latest attempts to capture Azaz and cut the Rebel's supply route towards Aleppo.
Well, they are the most organized, motivated and competent of the opposition groups in Syria overall, they will only keep getting stronger as long as the war rages on and the continued degradation of Syria as a nation and state.
Funnily enough Al-Qaeda in the Arabian Peninsula carried out the attack, and there a serious signs that this attack was planned since a quite long time