I understand that there are rare cases where a society can do nothing about a person but get rid of them forever

For the most part this purpose is served by the American prison system, where people who are put away for life are prevented from being a danger to society. Yet for those convicts who remain an extreme danger to life and limb despite being incarcerated -- a rare case, but it happens -- there may be no other option but to get rid of such people.

I am aware that this strict criteria doesn't factor into the actual criteria. Capital Punishment is usually applied to cases where the initial crime is so heinous that everyone thinks the perpetrator would be a danger, regardless of their incarceration. Or, more likely, it is applied as retribution in order for a society to spiritually cleanse itself after witnessing such horror. More cynically, Capital Punishment is applied to cases such as espionage and treason where the offense is against the power of the government, not against specific individuals.

As such, I have my doubts about the existence of Capital Punishment; while it seems useful in the abstract its continued use seems to rest on shaky moral grounds, if not outright tyranny on the part of the state. 

More importantly, though, I have my doubts about its execution, because the people who seek the death penalty and are involved in securing it are not required to attend the result. Not the judge nor the prosecutor, and the jury is long gone anyway, after ten years of appeals, and oh, the judge is busy, and oh, the prosecutor is busy. 

Phooey! Prosecutors already have wide latitude in seeking the death penalty. I say, if they want to decide who dies, they have to be the ones who do the killing. I want them to hold the damn sword instead of handing it off to some other slob. I want them to be fully responsible for their decision. I want them to look their target in the eyes before swinging that sword. It's too easy to sentence someone to death when you don't have to do the job yourself. I want these people to feel the full weight of what they've chosen. 

In short, I would like to create a deterrent effect against prosecutors who seek someone's death and then hide behind bureaucratic fooforal so they don't have to think about what they've done.