Suppose we acknowledge that "fundamentalist, extremist Islam is murderously violent," as you say. There can still be further questions about the social and psychological factors that lead young people to join it. Similarly Nazis can tell us what they believe in and are fighting for, but we can still ask, "Why was Nazism attractive to so many people?" Knowing what the ideology is and how it leads people to behave doesn't wholly answer the question of why people are attracted to that ideology and its movement.

That said, you do have a point. It's kind of disingenous of western society to feign ignorance of what could possibly make fundamentalism attractive. Mainstream western society, in its public face at least, tends to celebrate only a few rather flimsy and unconvincing purposes in life - money, fame, power. If you're the kind of person who finds all they look for in life under those categories, good for you. But if that's not you then you'll have a much harder time finding a purpose and, if you do find a purpose, remaining convinced that it's a valid one given that society doesn't overtly celebrate it (and may even call it foolish or a waste of time if it doesn't produce money, fame or power). If you're not a particularly subtle or patient person, as young people often aren't, then this leaves you with a painful gap or an aching self-doubt where the meaning should go. Crude ideologies with obvious promises of purpose can step into this void more quickly and readily than more truthful but less garishly impressive approaches to life. It's to be expected that young, energetic and impatient people will be disproportionately drawn to them.