I view this primarily from a utilitarian view. What benefit would come from executing him, versus what benefit would come from locking him up for life in a supermax without possibility of parole? Most of the studies I've seen suggest that there don't tend, as a practical matter, to be meaningful taxpayer savings for executions, thanks to the more extensive appeals processes. So, what's left?
The benefits I can see to locking him up for life:
(1) It deprives the Jihadist movement of a martyr. There's something a lot less sexy about a criminal who sits moldering in a cell until he's a decrepit old man than someone who goes to the gallows at his peak and can be imagined to be in paradise with his virgins. Thus, the life sentence is more likely to be a deterrent than the execution.
(2) There's always the possibility that at some future point he could be of use to society if he's alive. For example, maybe after cooling his heels in solitary for a year or two, he'll be so starved for excitement that he'll start spilling his guts about co-conspirators or secret terrorist networks, just for the human interaction. Or maybe in the future some brilliant psychologist will come up with a new theory about what really drives terrorists --a theory we could use to combat terrorism more effectively-- but in order to form and test that theory he needs some real, live jihadis of various types in custody, to interview and test (maybe there's even some physical brain defect that makes a person more likely to become a terrorist). Or perhaps he could even have a jailhouse epiphany and want to use the rest of his life to steer others away from terrorism, producing some valuable propaganda for us.
(3) Keeping him alive makes us look less bloodthirsty and may make it slightly harder to argue that we're the Great Satan. If we show at least a little mercy even to subhuman scum like him, it's harder to convince people that we're the ruthless monsters they make us out to be. And that may hurt terrorist recruiting efforts.
With those benefits to keeping him alive, and no clear benefits to killing him, I wish the verdict had come back in favor of life in prison without possibility of parole.
