There's a side of retributive justice that has not been raised, but is relevant. It isn't about punishment to the criminal, but closure to the public who endured the trauma of knowing people directly or indirectly impacted by the bombing.
I don't believe it's really working. They may feel better for a brief moment, but it won't bring back anybody to life nor heal sustained wounds. Beside, under American system, they will have to wait for years (maybe even two decades) before he is executed and many won't even live to see that. No hope for a closure, even brief one, for them.
I honestly don't see people impacted nor population in general benefiting from executing him (nor anyone else).