Why not the death penalty for child rape?

I'm against the death penalty as a state sanction, though perhaps for rather odd reasons. I think people have a perfect right to shoot robbers--once someone has implicitly threatened you, they've forfeited their right to the protection of the law. But I think that a state which commits cold-blooded murder is a brutalized state, and I have a visceral horror at the idea of putting a man in a cage and declaring to him the day that he will die. The process of executing criminals damages the moral fiber of all who are engaged in it, including the voters, a cost far in excess of the benefit to be gained from either deterrance or retribution. The former is dubious; the latter cold comfort. We strap a man to a gurney, pump corrosive chemicals into his veins, and then stand watching, awkwardly, while his life ebbs. All we get out of it is one more corpse.

But if we are going to have the death penalty, I don't see any particular reason to limit its application to murder. I can imagine much worse things than a quick, clean death.