I'm conflicted over the sentencing of Zacharias Moussaoui. (And if I didn't spell his name right, it is because I refuse to do him the honor of looking it up.) He was the only 9/11 hijacker stupid enough to get caught before he could do his special crime against man and God. The jury sentenced him to life in prison, because he didn't actually do that crime. He merely claimed to have had knowledge of it before it occurred, and that he was supposed to have taken part in it.
In some sense, killing is too good for him. If he had gotten the death sentence, we would have kept him in prison for years while we went through a complex and lengthy automatic appeals process, giving him numerous "days in court" to make a spectacle of himself and us, before finally offing him in the most merciful manner we know how. He deserves the misery of prison terminated only by whatever death God chooses, which might be a good deal less merciful than one of our lethal injections.
On the other hand, we don't need al-Qaeda people in our prisons recruiting other prisoners with shorter sentences and better American acculturation into new al-Qaeda gangs. Since we can't seem to socially isolate prisoners who are gang organizers, even in solitary confinement, maybe it is better just to kill them. You see, I oppose killing anyone who is already in the power of the state. But criminal gang organizers do not submit to the power of the state, even while in prison. They continue to organize, and even to order crimes committed outside prison. The same would be true of al-Qaeda members in our prisons. So, yes, we would be killing them for our shortcomings, our inability to control their anti-social behavior. But then, they could just stop that behavior if they wanted to live.
And yet, Moussaoui is probably not an organizer. He doesn't seem to have the snap to organize anything, not even his own thinking and behavior. In prison, Moussaoui is probably not a serious threat. So the jury let him live, made him live. It boils down to this - he was too stupid to avoid capture, and he is too stupid to kill.