This is prima facea evidence of why you should kill your attackers the first chance you can, because they are going to kill you anyways, and they know they can't get put to death.
(and probably think they'll get a pardon from Obama)
I think this case and others like it should again be brought before the SCOTUS. (it was a 5/4 decision)
Given the ages of the perps it is likely they could not even conceive of actual punishment happening to them, much less death. Their time horizons are probably not much more than a couple of hours. Talking to them about consequences to acts that will occur next year or next week is like talking Euskara to your five-year-old. It is simply not intelligible.