That Zacarias Moussaoui was given life in prison rather than the death penalty disturbs us much less than the jury's reasoning for their sentence. They, or at least several of them, evidently felt his life should be spared because he suffered an abusive childhood.
Very well, but if a miserable childhood means the terrorist doesn't deserve to die then why does he deserve to be imprisoned for the rest of his life? If he is the way he is for reasons he cannot help then why does he deserve punishment at all? The answer, an enlightened juror might give, is that he actually doesn't deserve to be punished. No one does. We are creatures at the mercy of forces we don't control, forces which mold and shape our behavior in ways of which we are only dimly aware, if at all, and we are not, therefore, in any sense responsible for what we do. Our behavioral decisions are really not our doing. We, like everything else in the cosmos, are little more than particles subject to the laws of physics. We are enchained and cannot free ourselves.
Then why imprison him at all, you might ask. The reply our enlightened juror would offer is that we need to protect ourselves from such as Moussaoui and prison is the best way to do it. Thus, no one in prison really deserves to be there. There is no such thing as guilt, at least not moral guilt. Prisons are just ways of protecting society from people whose determined choices have harmful consequences to others.
But if there is no guilt, if all of our choices are determined for us, why shouldn't society simply decide, as a consequence of the influences shaping its judgment, to execute the lot of them?