Peace requires something far more difficult than revenge or merely turning the other cheek; it requires empathizing with the fears and unmet needs that provide the impetus for people to attack each other. Being aware of these feelings and needs, people lose their desire to attack back because they can see the human ignorance leading to these attacks; instead, their goal becomes providing the empathic connection and education that will enable them to transcend their violence and engage in cooperative relationships.
Now, with regard to the people who have done things we call "terrorism," I'm confident they have been expressing their pain in many different ways for thirty years or more. Instead of our empathically receiving it when they expressed it in much gentler ways -- they were trying to tell us how hurt they felt that some of their most sacred needs were not being respected by the way we were trying to meet our economic and military needs -- they got progressively more agitated. Finally, they got so agitated that it took horrible form.