Wickerman correctly points to the risk of wrongly identifying the cause/culprit through an inclination toward over-simplification. The cause is extremist approaches to concepts and ideas, not a philosophy or belief system and certainly not the overall envelope of having ideas at all. To go from disagreement with a religion or philosophy to a view of "them" being identifiable (as an example) by one or more perpetrators of a crime wearing trousers (say, jeans just as an example) and extrapolate madly from there to then say that all who wear jeans not only agree with and support crime but are criminals is a fundamental mistake. The problem is that fanatical extremists (bordering on or actually straying into lunacy) are the ones who lose the plot, whether they are muslims, christians, nazi/political ideologues or atheists. It is in fact not the act of thinking that does it but taking a thought process up and along a destructive path and trying to convert reality to a deranged picture. Disagreement is one thing, violence based on it is quite another. We should be able to live with and rationally discuss the former, we should give no quarter to the latter.