There was an article lately about interviews with captured ISIS fighters on the death row. (I can't find the link unfortunately, maybe anyone's got it?) The idea was that what some of them describe is basically being forced to join the movement because there's no other way for them to provide for their families - they just suddenly lived in the middle of a war, but they don't care about the ideology itself. Biased? Of course, and doesn't apply to the European attackers. But still something to think about.