It's kind of a root and branch thing that I think those of us in the US have a difficult time comprehending. One thing I think that gets lost in this is that with the birth rate in Northern and Western Europe declining, there is a need for workers and those workers are coming from the Middle East. The problem is subsequent generations after the initial immigration are neither fully Middle Eastern or fully European and probably feel adrift. For all of their supposed open-mindedness, the French are notorious racists. Not sure about the Belgians, but I know that many of these terrorists come from ghetto-like communities within their respective countries.
That doesn't provide an excuse for terrorism, but I think it's pretty clear that the era of the melting pot is over in Europe and that the Middle Eastern immigrants are increasingly isolated within the European community and their atrocious behavior is part of that. Again, it's not an excuse for this crap.