Even the way sunnis and shia do their terrorism business is exactly a reflection of their beliefs. Shiites have always been about assasination and hidden terrorism because they are all about taghiyeh. They set fire and deny it, they kill people and deny it, they set bombs in synagoges, planes...and deny it, they promise you something and you can never bet on them to keep their promiss, they set you free with a smile and at the same time they pay people to kill you somewhere in the dark of the night, they laugh in your face and you never know if its real or not. You will never know.
The sunnis are much more naive and much more honest than this shia bunch. You easily can spot them and their goals. They kill people and admit it, they behead people and they admit it, they shoot planes and they admit it, they bomb shiite mosques and admit it and in case of shiites, they clearly tell you there is no chance they let you alive when they get you. so you as a shiite have only two clear choices when you face a ISIS fighter: fight and kill them, or they will kill you. there is no taghiye accepted, there is no gerye zaari o khod forooshi o doroogho daghal accepted.
You can see it from whatever angle you want, in any case we shites are the worse bunch because there is no clear path, no clear ghaanoon that protects you from the shiites. In shiitism everything is possible, no principles at all. Today you are friend, tomorrow you are an enemy, today its halaal, tomorrow a mullah declares it haraam. You never know what to expect from a shiite, everything can change from today to tomorrow.
Whats happening right now in our region is: the sunnis are fed up with shias and their aab zire kaah baazi o doroogh o daghal o fozooli in other peoples matters. Americans though like both sides for what they are: they brought up the shiite IR to achieve other, much more hidden goals and they brought up ISIS to achieve some different goals which shouldnt even be all that hidden to the public. So they are making use of the nature and natural tendencies of both groups to keep this region hostage.