I used to be a supporter of the death penalty until I really thought about my Christian faith and realized that it just doesn't fit with other things I say I believe, like believing that abortion is killing an unborn child, constitutes murder, and should be a crime.
It all comes down to hope. While there is life, there is hope.
While there is life, there is hope for the prisoner to recognize the error of their ways and make an attempt at apologizing to those whose lives they affected. For some families of victims, that is important. While there is life there is hope for the prisoner to recognize the error of their ways and make an attempt at setting themselves in better stead for the life we all have yet to come. For those of us who call ourselves Christians, giving that soul a chance at redemption is our highest and most important calling. While there is life, there is hope for the possibly-innocent prisoner to be cleared of the crime. It is disgraceful how high the rate of improper convictions in capital cases is in this country. While there is life, there is hope for the prisoner who was incorrectly given the capital sentence because they weren't really the trigger-man to get a more appropriate sentence.
Something to think about is whether or not the death penalty in its present form in the US is Constitutional. The reason is this: the average inmate on Death Row is there for many years before being executed, assuming that they are ultimately executed. Many aren't, because many ultimately have sentences commuted after lengthy and expensive judicial processes. But basically, the convicted on Death Row spends their day knowing that some day in the future, the People of the State of (wherever) will take their life from them. That knowledge hanging over someone's head would constitute torture in many judicial systems. Their crime may have been hideously inhuman, but their victim (or victims) did not have to live with the knowledge that some day, their life will be taken from them with nothing they can do about it. Or, if they did know that they would be killed before the act happened, they did not have to live with that knowledge for nearly as long as the convict on Death Row. It isn't making the punishment fit the crime. In many cases, it is making the punishment worse than the crime.
And now, some statistics. During the 1990s in the state of Florida, the average total cost to convict, incarcerate and ultimately execute someone ran approximately 5-6x the cost of convicting and incarcerating someone for life-without-parole. A convict on Death Row cannot be made to work on prison labor due to security regulations, but a convict with a life-without-parole sentence can still do something productive to offset (not repay, just offset) their debt to society. Some prisons, such as Angola in Louisiana, have farms that are worked by the prisoners.
If we say we value life, and we do say that by saying we are against abortion, then we must not quibble over which lives are worth something and which ones are not. We are either pro-life all the way, or we are not pro-life but merely anti-abortion. If we are pro-life, then we are FOR the life of the unborn child, FOR the life of senior in the hospital, FOR the life of the disabled or different, FOR the life of the prisoner, FOR all human lives because they ALL have value - if for no other reason than that they were all Created by the same God that made us all and loves all of His creations.