It is said by many that Islam is a religion of peace, and that the Quran is misunderstood by those who think otherwise.  But it is not what the Quran actually says that counts, so much as how the Quran is interpreted by those who claim to follow it.  Too many of those who claim to follow the Quran see Islam as a religion of violence, and they act accordingly.  The Quran supports them; it says:  "But when the forbidden months are past, then fight and slay the Pagans wherever ye find them, and seize them, beleaguer them, and lie in wait for them in every stratagem (of war); but if they repent, and establish regular prayers and practice regular charity, then open the way for them: for Allah is Oft-forgiving, Most Merciful."  One can see how a terrorist would find support in the Quran.