My husband's brother is living in Paris, under very different conditions to my husband here in rural Norfolk. He lives in the banlieux, where everyone is a Muslim, nearly all are fundamentalist, and many living illegally in France (he overstayed his Visa ages ago). He is far from 'integrated' and during the Paris attacks it emerged he actually lives with his African mates in St Denis, near the Stade de France. Such a man is very vulnerable to being 'recruited' by ISIS. He has nothing to lose. But none of this is any excuse whatever to decide to murder innocent people in vicious and evil bomb attacks. There can be no justification for that ever.
Our village has always been delightfully welcoming to my husband. He has often walked into our local pub (soft drinks only!) and been hailed by one and all.
I feel that the only way forward is to vastly increase surveillance and 'Intelligence' by the relevant Departments, and to keep tabs on all possible suspects. There must be known leaders and recruiters who should be watched and tracked. Brussels seems to have fallen down a bit on that side of things.