People are getting too caught up with assigning motives and justifications for atrocities such as this. There is absolutely no justifiable reason for slaying innocent civilians, ever. It doesn't matter what can be cited from history, either recent or ancient, it can not be justified and it can not be allowed. Those who perpetrate such evil against the innocent need to be eradicated. The cultures, governments, and religions of the world have their differences, and often clash with each other. Sometimes this leads to war. As terrible as war is, it's still not a just cause for murdering the innocent, and should never be tolerated, no matter which religion the innocents embrace, nor in which country they dwell.
ISIS needs to go away, permanently. This is something that the entire rational, civilized world should be able to get behind.