It is hard for most people steeped in the humane, liberal values of Western Civilization to understand the massacre of innocents. To slaughter to make a political point. But terrorism is not likely to disappear. Indeed, it is a surprisingly common practice. Although Americans were taken unaware on September 11, many other peoples have long suffered from the murderous attention of domestic and foreign terrorists.
Terrorism is common, and will persist, because it is a tool of the weak versus the strong, a cheap military weapon to achieve expensive political goals. As long as there are people willing to kill to advance their ends, there will be terrorists.