I agree, and even some of the terrorists in the Paris attack were French citizens. But it's also important to recognize that the ideology that influenced the attackers was not domestic. We now know that most likely one of the attackers got into Europe as a refugee from Syria, there may be more but we don't know as of now. As I said before, I believe moderation is key.
I agree with DGFone, this is one of those real world situations in which both decisions are mistakes. To deny the refugees would be to end their chances at a decent life, but to accept all that came would only increase the chances of foreign terrorists entering Europe, as well as infuse and domesticate misogynistic, non-progressive, third world cultures into the Western world.
That's another thing, the cultures many of these refugees are shaped from are perpendicular to many Western ideals. Just to show a few statistics :
84% of Egyptian Muslims support the death penalty for leaving Islam
86% of Jordanian Muslims support the death penalty for leaving Islam
30% of Indonesian Muslims support the death penalty for leaving Islam
76% of Pakistanis support death the penalty for leaving Islam
51% of Nigerian Muslims support the death penalty for leaving Islam (http://www.pewglobal.org/2010/12/02/muslims-around-the-world-divided-on-hamas-and-hezbollah/)83% of Pakistanis support stoning adulterers
78% of Pakistanis support killing apostates (http://www.realcourage.org/2009/08/pakistan-78-percent-call-for-apostate-deaths/) 81% of Egyptians want strict Sharia imposed in every Islamic country
76% of Pakistanis want strict Sharia imposed in every Islamic country
49% (plurality) of Indonesians want strict Sharia imposed in every Islamic country
76% of Moroccans want strict Sharia imposed in every Islamic country (http://www.worldpublicopinion.org/pipa/pdf/feb09/STARTII_Feb09_rpt.pdfI could go on, but these statistics are not that difficult to find. The point is this isn't simply immigration from Canada or the United States, where many of the same fundamental ways of life are shared (such as the right to freedom of speech and freedom of religion), these people are often shaped by a culture that is entirely contradictory to ideas that modern liberalism prides itself on.