There are two main explanations for why some Islamist extremists want to terrorize the western world, each with implications for public policy.
The first was spelled out after 9/11 by U.S. president George W. Bush: "They hate our freedoms: our freedom of religion, our freedom of speech, our freedom to vote and assemble and disagree with each other."
More recently, Stephen Harper has made the same point. Canadians "are targeted by these terrorists for no other reason than that we are Canadians. They want to harm us because they hate our society and the values it represents."