If the death penalty is necessary to keep order, then it should be recognized that nothing is perfect. If it is not necessary to keep order, it should be abolished. Cruelty is a sign of desperation and weakness. A healthy, stable society has no need for the death penalty. A society that is stable, yet retains the death penalty, is sadistic, and just as bad.
It's also simply completely backwards and idiotic to abolish the death penalty for crimes where it would be a deterrent (economically motivated ones such a robbery), and retain it only for the insane, whom it doesn't deter. Yet that's what our society has apparently decided. It's almost as if it's desperately lashing out against the reality that there's no such thing as free will, since the second class brings that reality explicitly to the forefront. The only point of free will, after all, is to find others guilty. And how would we reduce them to human shaped objects, useful for only the pleasure that their suffering brings us, otherwise?