The "Islamic State" as the group is now called, is a political reaction not a religious movement. Yes, they all belong to the same sect of Islam, but their war isn't holy. They are a group formed to fight against the Asad regime in Syria and the American-installed puppet regime in Iraq (which is why the group was originally called "Islamic State of Iraq and Syria").
We as Sunni Muslims believe in the prophecy that describes the return of an "Amir el-Mu'mineen" (Khalifa) which will pave the way for one unifying Islamic State , however this prophecy comes with many detailed events that have not transpired with the installation of this new "Islamic State" and this new "Khalifa", Abu Bakr al-Baghdadi.
Firstly, the line of Khalifa will begin with the Mahdi who's name will be Muhammad Ibn Abdulaah. This man will appear during his pilgrimage in Mecca where learned men of the faith will witness him and insist he is the Mahdi. He will deny such a title and will be chased down, insisting he is not worthy of such a title.
Today, this self-proclaimed Khalifa, Abu Bakr al-Baghdadi, is just another militant who has created his own group.
Secondly, the Mahdi will return to unify the Muslims, not divide them. He will bring peace and justice, not death and destruction. He will protect minority groups such as Christians and Jews, not displace them from their homes.
I must admit, I was excited to hear about the birth of this "Islamic State" until I saw what they were capable of, and the majority of my fellow Sunnis agree that we will still have to wait a little while longer for the return of the Khilafa.
Ultimately it all comes down to one simple fact: Militant groups like the "Islamic State" and Jabhat al-Nusra will always emerge when the Muslim world is denied its rights and freedoms. When dictators and puppet regimes are installed to enforce their own laws violently with the help of external Western influences, we can expect the response to be just as violent.
What could we possibly expect from the oppressed nations of Iraq and Syria but an emergence of reactionary groups who respond in kind?
Many Sunni Muslim do not support "Islamic State", but we can certainly understand why they exist.