Slavoj Zizek takes a stab at answering the question:
So why are we witnessing the rise of religiously (or ethnically) justified violence today? Precisely because we live in an era which perceives itself as post-ideological. Since great public causes can no longer be mobilized as the basis of mass violence – in other words, since the hegemonic ideology enjoins us to enjoy life and to realize our truest selves – it is almost impossible for the majority of people to overcome their revulsion at the prospect of killing another human being.
Most people today are spontaneously moral: the idea of torturing or killing another human being is deeply traumatic for them. So, in order to make them do it, a larger "sacred" Cause is needed, something that makes petty individual concerns about killing seem trivial. Religion or ethnic belonging fit this role perfectly. There are, of course, cases of pathological atheists who are able to commit mass murder just for pleasure, just for the sake of it, but they are rare exceptions. The majority needs to be anaesthetized against their elementary sensitivity to another's suffering. For this, a sacred Cause is needed: without this Cause, we would have to feel all the burden of what we did, with no Absolute on whom to put the ultimate responsibility.
A few things. First: the banality of this observation is its most striking feature. Second, I haven't seen a big increase in Christian or Buddhist violence. The issue is primarily with extreme Islam, which in turn has provoked reactions in Africa and elsewhere. Third, how many "spontaneously moral" people do you know? Me neither. Slavoj Zizek must be one of the most over-rated pseuds out there right now.