@Aurelito: Wait, what?
I really hope this is a joke. You DO realize that we imprison A LOT of innocent people, right? Just look at the West Memphis Three. They were given life for a murder they didn't commit and it took years and years before they were released, despite massive amounts of evidence proving their innocence, just because their local law enforcement didn't want to be embarrassed.
Prisoners should never be murdered unless their crime was so impossibly heinous that society cannot move on from it.
But there are political problems too. Mainly, for some well known prisoners, killing them would make them a martyr and inspire more people to lash out because of their death.