Dostoevsky rightly observed that if there is no God or eternal life, man can justify any action. This is why communism has brutalized and murdered so many.
Despite believing in a creator and in eternal life, the Jehadis have not had religion stop them from justifying any action.
You, and Dostoevsky, for that matter, are assuming that Atheists are maniacs who would love to do anything without any fear of consequences, and with wanton abandon. The truth is, Atheists fear chaos, too. Any situation that encourages anarchy, also threatens the Atheist, and his or her descendents. That in itself, is reason enough to seek order, on a pure, Darwinistic level.