Also, who is allowed in is important. Soul-dead folks from war-torn African or ME nations are not going to assimilate well, on average.
It's not the Quran (sorry guys, but you need to read your Bibles without your clergy's goggles on). It's that Europe's empires left shitholes, and as those shitholes started to come together, their successors tried to, "fix things," left and right, and screwed it up even more, mostly during the cold war (not that we aren't still cycling those personalities through our circles of power). The cultures left from all that seem to have, by and large, coalesced under Islam.
One thing the violent factions seem to have in common is acting like abused: the idea that abusing unrelated people now is actually a worthwhile revenge tactic against actions from the past (FI, if they wanted to assassinate someone near the top from that time, regardless of the ethics of it, that would at least be revenge against a related party).
I do not mean to defend the swaths of supportive and apathetic Muslims, but it is pretty clear that it is a culture and class problem (there's more to learning than just the 3 Rs: emotional habits are also learned), more than anything else, and no appeasing or kind supporting will take care of it (what will, aside from deportation and mandatory cultural assimilation, to weed out the self-ghettoing types? Hell if I know).