Dave Betz summarizes one explanation for terrorist motivation:
Bonding capital is inwardly focussed, it is ‘good for undergirding specific reciprocity and mobilising solidarity’; it is about reinforcing exclusive identities and ‘by creating strong in-group loyalty, may also create strong out-group antagonism.’ Take the picture [above], for instance, which shows the young American Nick Berg moments before the men behind him, one of whom is thought to be Abu Musab Al Zarqawi, saw his head off–all captured on video and proudly distributed.
Now consider [the] question ‘what do terrorists really want?’ Indeed, what do they want? What is the purpose of killing in this particularly gross and degrading manner? As a normal human being it’s ‘incomprehensible’ and best leave it at that; except probably I’m not normal…because I think we ought not leave it at that and that the act is comprehensible. It’s an extreme example of the building of bonding capital. Can you imagine a more explicit drawing of the line between in-group loyalty and out-group antagonism? Me neither.
The Dish 