I'm not really sure how I'd feel about an ideal death penalty: one that was applied fairly to everyone, rich and poor, black and white; one that never got an innocent man killed; one that was administered in a clean, painless, humane way.
Think of it this way for a moment: where does the buck stop?
Morally speaking, the death penalty is unjustifiable at best, and a monstrous imposition on the part of the state - it makes its employees take human lives in its name - at worst.
You see, morality does not concern itself with context. Stealing is wrong, no matter how hungry you are. Lying is wrong, even to save your life (Christian martyrs exemplify this ideal in western society, I think). And killing is wrong even when it's for self defense - let alone for some vague concept of public revenge.
Anyone who takes a life is guilty of the same crime - taking what they have no power to give back. You can call it murder or you can call it the law, or even just deserts - but the upshot of taking a human life remains the same.
Retaliation is never the productive way of resolving a conflict. Violence begets violence. These ideas are so familiar to us that they seem like meaningless platitudes, but we ignore the guiding truths behind them at our peril, both on the personal and the macro-social scale.