Culture, but since every culture itself has so many different factors that contribute to its nature, I would say that economic disenfranchisement and lack of education (which of course play into each other as well) would be the most blatant contributors to the violent extremism. Religion simply becomes the vehicle by which to justify and perpetuate the violence.
As we all know there are completely wacko religious zealots right here in the USA. My personal opinion as to why they aren't violent is not their religion, but the fact that it's hard to be angry enough to blow up your own cranium when you have two cars and a big screen LED TV. When your station in life doesn't suck dog balls, it's hard to justify the anger and desperation that drives the violent extremism.
Just my .02