The execution of anti-abortion nutcase Paul Hill shows very well the futility of the death penalty. Hill was a deeply disturbed individual and a danger to society. He needed to be locked away from society, but there was no reason why he or anyone else should have been executed.
The death penalty serves but one and only one purpose in our society - vengance. It accomplishes nothing else that cannot also be accomplished by life imprisonment. It does not deter other people from committing crimes and it allows no room for correcting errors when an innocent person is put to death.
I think it is sadly ironic that a nation that claims to be overwhelmingly "Christian" is so strongly in favor of the death penalty. After all, Jesus was himself a victim of a state-sponsored execution. Jesus flatly rejected the "eye for an eye" values of the Old Testament and even stood up to a mob at one point to stop an execution. When Jesus prevented the stoning of Mary Magdalene, he did not do so because he thought she was innocent of the crime. He defended her despite her guilt. Forgiveness was the key to Christ's message and we are a nation that is very short on forgiveness.
There are many ways to argue about the death penalty - both secular and religious. But I think it all comes down to a person's philosophy of life which can be boiled down to the simple question "Why are we here?" If you believe as I do that we are created by a loving God then you have to then ask what is it that he expects us to do while we are here. The best answer that I can come up with is to love God and we do this by loving his creation - i.e. one another. The more we love one another, the more we show our love for God and the closer we come to God. A very simplistic formula, I know, but it works for me.
Assuming that God wants everyone to reach this goal while they are here and knowing that God is very big on forgiveness, I have come to the conclusion that we should not be in the business of shortening the lifespan of people who are probably the furthest away from reaching that goal. Maybe this person would have turned their life around in prison when they were 40, but we executed them when they were 25. Maybe some people think this is just because out of a need for vengance they want this person to be separated from God (or in their minds "Go to Hell) rather than have the opportunity to turn their life around (after all, their victim(s) did not have that opportunity). But don't you think that God takes all of this into consideration? Wouldn't a loving and forgiving God be more likely to give someone a second chance when their life was artificially shortened by execution, murder, war, plague or accident? It seems only fair to me.
That is why we need to get out of the business of executing criminals and leave matters of vengance up to God.