I really should know better than to get involved in this thread...
But i think the main issue is, whenever there is an act of terror which appears to have originated from within the Muslim (world) community, there appears to be very little condemnation from within the powers-that-be (such as they are) within the Muslim religion.
Take for example the beheading and subsequent recent issues in France... i stand to be corrected, but i haven't seen many examples of community leaders from across the Muslim world (either in France or elsewhere) condemning the actions. You could say "well they aren't responsible for every extremist nutter...."
But it would go a long way to dispelling the opinion which seems to be widely held, that significant sections of the Muslim world, at best understand why these acts were carried out, or at worst, believe they are justified.