Who is to blame?

Unfortunately, the answer to "who is to blame" is not simple. One perhaps useful way to think about responsibility for terrorism is as a series of concentric circles, each less causally immediate as you move out from the center, but each also more all-encompassing and therefore harder to address. 

The inner circle is the immediate killers, their personal psychology, experiences, etc. The next circle are the (in this case jihadi) violent ideologists, as well as their tacit supporters. The next is the intolerances and lack of assimilative mechanisms in the (in this case) historically non-Muslim societies in which many Muslims find themselves implanted (often by birth rather than individual choice). The next is the failure of some Muslims to reconcile their religious faith to the exigencies of a global system which is predicated on information openness and which includes many liberals, and indeed many Islam-haters -- who Muslims cannot avoid seeing and hearing. In the next circle out we find the unjust and indecent historical practices by the West against the non-West (including "the non-West at home"), which obviously includes but is not particular to the treatment of countries and peoples in the Islamic world. 

No doubt one may adduce other layers, and certainly one can debate the order of the circles: they overlap in non-concentric orbits, so that different ones are prior in some places than others: the "blame" for terrorism is very different in Borno than in the banlieues. The key point, however, is that, depending on which of these circles one privileges, one can come to very different conclusions about who to blame for particular acts of terrorism.

An essentially historical method, this approach is almost certainly not amenable to quantification, but it does have the virtue of explaining why different people of decent intent and intellectual integrity can come up with highly divergent interpretations of responsibility for terrorist atrocities. This same mode of analysis can be used to assess responsibility for other kinds of terrorism too, from Breivik and McVeigh, to ETA and the IRA, to today's Naxalites and anarchists at the turn of the 20th century. 

As always, where you stand depends on where you sit...