ISIS is a cult. One should no sooner call ISIS a Muslim group any more than one would call Charles Manson's club a Christian group, even though Manson's theology revolved around the Christian bible and its prophecies.
In fact, most of ISIS's members are in the same age group as Manson's were, and as most people in cults are. Gullible losers. That's all they are. And these dipshits who are committing violence in ISIS's name aren't even members of ISIS. They are mental cases who want to die and go out with a splash. That's all THEY are. They want their name in the news, and so they scribble a little note pledging their love of ISIS because they know it will make headlines.