I just cant imagine what sort of upbringing could lead to a mass of people who think that murder and war are the first and best options. This is a religious war but their religion isnt islam, its violence.
I recall being taught about the victories of Islamic conquests through the ages starting with the Muslim-Quraysh wars. In Islam, war is seen as a reality of human existence, and as a valid means to deal with perceived injustice, oppression, etc. Finally, martyrdom in Islam guarantees one into paradise...
The problem with Islam is that it's extremely malleable. You can pick and choose your sources, scholars and books and have an Islam of your own. Each one has their side, and accuses the other of taking things out of context, or having an incorrect understanding. Some Muslims make heavy use of the No true Scotsman fallacy.
The former allows one Muslim to basically slide on the scale, from a nominal Muslim to an member of ISIS. You just need someone convincing enough to show you a few prophetic traditions, sound charismatic enough and revive your faith in the religion.
What religions don't have this issue? Using the most popular religion in the US, Christianity has hundreds, if not thousands, of branches that all say they have the "right" version of things, and everyone else is at least slightly wrong.
I'd say it's an argument against any/all religion, really. They can all be devolved into a message of hate and violence.
Like so many other things the concepts of "belief" and "faith" are watered down in common usage, but some reflection on how one might reasonably behave given (i.e. taken axiomatically) knowledge of God's nature, God's will, and eternity reveals such a state to be an awe-inspiring, terrifying thing. I find true faith to stretch the bounds of understanding, so powerful and alien it is.
It is fortunate that the majority of believers in the Western tradition so thoroughly de-claw their religions, excising or conveniently ignoring the parts that may require of them great daring or enormous sacrifice, or don't consider what true faith in matters of an eternal nature implies, or we may expect to be inundated with pauper-hermits, wild-eyed street preachers, and suicide bombers.
It would be a very strange world where your typical believer placed as little value on the things of this life as their Books suggest they should, and where matters of conversion of non-believers, proper thought and action, and obedience to God truly resonate across eternity in their minds. It's the kind of thing that can make mass-murder of the already damned entirely reasonable if it saves or protects the eternal souls of even a handful of others.
[EDIT] See also Kierkegaard's Knight of the Faith, https://en.wikipedia.org/wiki/Knight_of_faith
And to be clear, I don't intend to demean meeker forms of religious expression, but I think it's selling the power of religion entirely short to say that extreme, even violent behavior is somehow other and doesn't count as legitimately religious, which some seem to insist on doing (e.g. "they're not real Muslims", "they've twisted the faith", et c"). To take it a step farther I would say your average ISIS member is likely much more authentically religious—or at the very least so much more dedicated that they're almost in a different category entirely—than your average Sundays-and-Christmas Christian, for example, to the extent that comparing authenticity-peens is even valid—though if it's not then how can anyone say ISIS' expression of Islam is illegitimate?
http://wartard.blogspot.co.uk/2014/09/the-idea-of-isis-histo...
http://wartard.blogspot.co.uk/2014/10/the-west-v-isis-air-st...
You will also find the new adam curtis film, Bitter Lake, informative.
TL;DR its not just to do with religion, its to do with history, and a lot of stuff.