See other issues.
We believe that the death penalty is an important tool of society that has been made too costly. We do not believe that we need to abolish the death penalty to save money, but rather that we need to make the death penalty less expensive.
A common argument against the death penalty, is to point out that it is not technically possible to avoid killing innocents on occasion.
Ironically enough, abortion, which guarantees the murder of innocents, has been credited with reduced crime rates in New York, and many think this makes abortion a good thing. Belief in the goodness of abortion, however, correlates strongly with belief in the badness of the death penalty, often attacked for its killing of innocents.
We consider abortion to be a form of murder, but consider it *potentially* justifiable under a very narrow set of circumstances, namely threat to the life of the mother, threat to the life of the child, or when the pregnancy was a result of rape. So, I would occasionally condone the killing of innocents, sad as it might be.
Similarly, in war, there is a risk of killing "innocents" (non-enemies), but the risk of killing innocents does not lead me to conclude that my country needs to abandon defense of its citizens. (Though, war is, often enough abused.)
If I was to use a gun to defend myself from an attacker, I might accidentally hurt myself, or someone else. Do I then abandon that self defense?
Similarly, in each circumstance where we might consider applying the death penalty, there is some risk of killing innocents. The death penalty, however, is a form of domestic self-defense (or, at least, it should be thought of that way), and while the opportunity for reflection should be embraced, so as to come to a thoughtful determination of the best course of action, the fact remains that we are weighing very real risks and costs against the risk of killing an innocent, and just like in other scenarios, that risk, and that cost, is not so great as to justify paralysis.
Killing to Show Killing is Wrong
It is possible that capital punishment has some deterrent affect, but that should not be a significant concern. The "tough on crime" viewpoint has been shown to do little more to deter crime than sentences and policies of a softer nature. It has lead to high recidivism rates and cruelty.
We don't (or at least shouldn't) kill people who kill people to teach anybody that killing is wrong. Killing people, in fact, isn't always wrong, so that would be a stupid idea to teach.
Killing is necessary to sustain life. Death is a natural part of the human experience. We kill people who are "murderers", because they pose an unacceptable risk and burden to society by demonstrating a willingness to violate the right to life of others without reasonable cause. (eg. Self-defense, defense of property, defense of country, defense of neighbor, ...)
The argument dujour against the death penalty revolves around the fact that it is a very expensive punishment to inflict. In an attempt to thwart the assertion of opponents of the death penalty who point out the fact that innocents get killed, we have created a process which is onerous in the extreme. In doing this, opponents of the death penalty were given yet another argument against the practice. The death penalty should not be expensive and it should not take even one decade to execute. However, even in the face of such expense, it is clear that "allowing" the death penalty, as we do now, is not the same thing as "requiring" the death penalty. When the death penalty is removed as an option, it means that such action can never be taken, no matter how beneficial it may turn out to be in some particular case. With the death penalty left as an option, we are not forced to take take upon us that expense in any particular situation. Thus, forbidding the death penalty does not necessarily save us money, but does necessarily reduce our options for dealing with criminals.
It might make sense to define guidelines that prevent the use of the death penalty when the cost is not worth the benefit, but to do away with it altogether seems foolish.