Often, schoolyard bullies attack peaceful bystanders of high status, if they think they can get away with it. Often, they even think of some justification for themselves. I think the same principle applies here.
Exactly. Al qaeda is not some freedom fighter movement, they are fighting to create a new world order. An order where western powers no longer interact with the islamic world, and are also ideally disempowered and lowered in stature. An order where the islamic world is united under theocratic control (a new caliphate) and ideally is the dominant power on Earth with all other races, religions, and cultures their inferiors.