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.
An atheist may choose to deny the existence of God, while pragmatically choosing to adhere to codes that have sprung forth from the idea of God and immortality.(they are the only ones that endure)
Even Richard Dawkins, in his book "The Selfish Gene" admits that humans are born selfish as a biological reality. He proposes the need for ethical training to "learn generosity and altruism." What he can't admit is that if a human is just a biological being. . .it doesn't make natural sense for a human to act in contrary to his biological nature. . .this was Dostoevsky's point. This is what George Washington meant when he stated in his farewell address, "reason and experience both forbid us to expect that national morality can prevail in exclusion of religious principles." The "religious" view of a human is that we are not just biological in nature. . ..we are also spiritual. It is the spiritual component. .."God's image" that then makes us worthy of the "endowment" from our Creator. . .of liberty, life and the pursuit of happiness.