I read this piece from The Atlantic, What ISIS Really Wants. It is a really long article that will take you some time to read, but worth the effort.
It changed my thinking significantly about the type of organization we are dealing with. While in modern terms we consider them terrorists by their actions, if the story is true they have theological reasons for doing what they do. It's very medieval in its nature and absent of any sense of modern morality.
The abominations that have been done across the ages in the name of religion are many. Unfortunately, it appears that in the Middle East they will continue for some time to come.
The one bit of good news I heard from another source today is that Christians and the West in general are a low priority. That is little comfort to the people and nations living within and in areas contiguous to this self-declared caliphate.