The man was 17 1/2 when he committed the murder/carjacking/attempted murder crime (he killed the driver, shot but did not kill the passenger, and stole the car). How much "smarter" was this thug going to get in 6 months? We were told how he was elected class president, how he was a star athlete.
This is just another trick in the bag for lawyers to keep the guilty from getting their due (not a saving by grace, but a loophole).
There have been other unusual cases like a man who was fine when he committed the murders but sustained some kind of head injury later (possibly a self-inflicted gunshot from an attempted suicide). Some claimed that it wasn't right to kill him since he had no idea what was happening to him or why.
I heard that this ruling is bad also because it purposely left the determination of mental ability/retardation to each state meaning that they won't have a common definition.