There are two views of the world. In one view, one accepts that human beings are capable of horrible things as well as wonderful things, while using their individual beliefs to justify extreme behavior on either side and all those in between. In the other view, human beings are capable of horrible things as well as wonderful things, but what marks the difference depends on how much they agree with your own beliefs.
Take the latter too far, and Andrew Sullivan explains what you get:
Both Islamism and Christianism, to my mind, do not spring from real religious faith; they spring from neurosis caused by lack of faith. They are the choices of those who are panicked by the complexity and choices of modernity into a fanatical embrace of a simplistic parody of religion in order to attack what they see as their cultural and social enemies. They are not about genuine faith; they are about the instrumentality of faith as a political bludgeon.