I hate to say it but I just think it is human nature. Violence and greed have always been around and religion has offered a convenient excuse to practice it. Religious fanaticism, from the Spanish Inquisition and Salem Witch Trials to Jihad, prays upon those who have the least to lose by buying into the rhetoric. It can give those who feel they don't belong a sense of belonging, it can give those whose lives are not that great in the present a hope that the next life will be better, along with many other reasons that only the person who chooses that path can tell you. Of course it should be noted that fanatacism is about the follower of the religion and not the religion itself. I also believe that there are those that deliberately twist religious tenets to further their own cause and greed and entice those who are ripe for the picking to join their cause in the name of Holiness. This is just my humble opinion.