It is wrong to say "ISIS are not Muslims" and it is extremely unhelpful to separate them from the religion. My tutor actually has spoken on national TV and written articles about this exact topic. He is a Shi'a Muslim and an academic, and he argues - quite correctly I think - that if you ignore the religious roots of the group then you cannot possible grasp the problem. Because their ideology, their beliefs and their objectives, are entirely religious. They fit within a framework that is Islamic (albeit a distinct brand of fundamental Islam) and their justifications are entirely theological.
If you disassociate them from Islam, then you have to explain their motives and actions by completely different terms. This is something you hear a lot: 'They just don't know how great Western culture is'. 'They are poor and marginalised so turn to violence.' 'They are responding to the US occupation of Iraq.' 'They are responding to European colonialism.' 'It is all about oil'. So on and so forth.
Some of those things have elements of truth - marginalisation, poverty and retribution certainly are causes as well. Yet the biggest cause, above anything else, is their religious belief. If you are an atheist like me, you can only truly understand this by imagining how you would see the world if you were a fundamentalist Muslim.
Once you do that, (and it requires a basic understanding of fundamental Islam that I don't have time to write here), then it all makes sense. It works the same for if you imagine you were a fundamental Christian - this might be easier to imagine.
If I believed that the world was going to end and I had to obey the law of the all-seeing, all-knowing, all-powerful deity in order to reach eternal paradise, I'd do whatever the hell was needed to get on his good side. If that means killing people, why wouldn't I? This world is just a temporary, physical one. It's worth it for infinity in paradise. And they are non-believers anyway, they know nothing.
If that is how you see the world and that is how you understand it, then these acts of violence make sense. The whole Islamic State makes sense.
Where it gets extremely tricky and sensitive is how non-fundamentalist Muslims fit into the picture. The same for non-fundamentalist Christians, or Jews. Because the fundamentalists would argue, and in a way I agree with them, that the beliefs of these people are so far removed from the original message and meaning of the religion that they are not truly Muslims, or Christians or Jews. In order to achieve a form of Islam, or Christianity, or Judaism that is acceptable to 'Western society', you have to reshape and twist the doctrine of that religion SO MUCH that it can start to not make sense at all.
Christianity is the perfect example. I live in Britain, which is a former Christian, now secular country. The majority of people are atheist - the Church has lost most of its power and influence. I think that this happened because the Church in this country was forced to adapt to the new ideals that came out of the Enlightenment. By doing so, over a long period of time, the doctrine of Christianity became so divorced from its scripture that it stopped making sense. As a schoolchild, I was made to go to church twice a week. The priest would tell us that Christianity preaches equality, freedom and love for everybody, including people from other faiths. But then we would go and read the Bible, and it didn't have that message at all. It told us to commit genocide on people of other faiths. It was violent, and brutal, and had so many historical problems with it that it was hard to believe. The religion didn't make logical sense any more. The result of this was a generation of people turning away from Christianity, and now you have a secular Britain.
To a much more limited extent, the same is happening to Muslims in Western countries. Many of my friends are Muslim. Yet they don't pray 5 times a day. They don't have multiple wives. They follow our legal system, not shari'a (there are a lot of misconceptions about shari'a, but that's another story). Why? Because this is how they had to adapt their religion in order for it to fit within a Western framework.
So many of them would read the Qur'an and the Hadith collections and realise how far removed they were from the fundamentals of the religion. Western Islam has to reinterpret and abstract the scripture so much in order to remodel the religion as acceptable to post-Enlightenment ideals, that it no longer makes sense to a lot of Muslims. Many turn away from religion entirely and become atheist. But many go the other way, and begin to follow the scripture fundamentally. These are the ones who, in the west, ~~turn to groups like ISIS.~~ are more likely to turn to extremism and violence (although this not always the case).
That is why it is unhelpful to say these terrorists are not Muslim. If you do so, you cannot discover any of what I have just said. You limit your understanding, and you actually make it *easier* for the discourse to become 'us vs. them', rather than peaceful and loving as it should be.
I hope that helps, I don't normally write these sorts of things on Reddit because nobody on /r/worldnews is intelligent enough to grasp concepts beyond "us and them", "Muslims r bad". I would truly suggest learning about Islam - we in the West are disgustingly under-educated. I don't know everything, but having learned the theological and political history of Islam and the Middle East, I am constantly frustrated at how little people know and how uneducated their opinions are. It has a beautiful and rich history, and there are misunderstandings and misconceptions around every corner.
**TL;DR: Those who disassociate ISIS from Islam and say they are not Muslims are wrong, and this argument makes it impossible to truly understand their motives and objectives. However, the other side, which argues that all Muslims share these motives and objectives, are also wrong. The real answer lies somewhere in the middle.**
**Fundamental Islam IS incompatible with Western-style liberal democratic society. But so is fundamental Christianity - that is why much of Europe has turned away from the Church and towards secularism. It is not just Islam. It is all of these religions, with severely outdated doctrines and dogmas, that are incompatible.**