There are plenty of parts within the Quran that state violence is warranted when either under provocation, oppression or in self defence. Depending on what views you subscribe to ultimately defines what constitutes provocation, oppression and self defence. Some Wahhabists believe that violence is justified in order to bring the world under the rule of the worldwide Caliphate, whether it is provoked or not. I'm not saying any of the posters here are of that persuasion, but there is most definitely room within the Quran to justify violent acts. It all comes down to interpretation and that is what is wrong with religion.
EDIT: I may or may not have got this confused with Salafis, but they are quite similar