Because the terrosrist have a been Muslims so tempted to believe something is wrong wth the religion that causes young men and women to kill themselves in the process of riddling the world of a few infidels.
However, as it is becoming more clear the vast majority of the followers of Islam did not approve of such terror. There are at least 1000 peaceful Muslims for every violent one, thus the answer probaly does not lie in exploring the Muslim faith looking for the causes and murderous behaviour.
Rather there must be something in the minds of these suicide bombers causing them to sacrifice their own life kill random strangers, many of their victims never gave second thought to Islam.
Islam like we all known if praticed properly is the most peaceful religion on earth, some might decide to go against the religions will making the world think the religion in responsible for the many terrorist attacks in the world today without taking a proper look at the terrorist and his/her personal vendatta.