I do not support capital punishment.
1) The State which has so much power is ever greedy for more, and the more it gets, the more it wants. There is the very real danger that capital punishment will be extended to more and more offenses if it seen as a solution to what we now consider "capital" crimes.
2) There is no real need for it when life imprisonment is available. If the justice system were more truthful, then a life sentence would really mean no parole, no furloughs, no anything but a life inside a two by three meter cage.
3) The threat of capital punishment can be used to coerce an accused innocent to confess to lesser charges.
4) Capital punishment puts the United States in the company of countries like Communist China and Saudi Arabia that have no respect for human rights.
5) Some innocent may be executed; while I think this is relatively uncommon, it can still happen. As evidence, I point to the numerous persons on death row that have been exonerated by DNA evidence. How many others might have been set free if the technology had been available sooner? What if there is yet undiscovered technology that might save any innocents that are on death row today?
6) It is incredibly expensive. The costs to the taxpayer for all the trials and appeals is something like three to ten times higher than for a life imprisonment penalty. That money could go towards feeding hungry people or helping the homeless. Should the State allow an offender to further victimize others in this way?
7) It is a dubious deterrent at best. To all the guilty on death row, it was no deterrent at all. To the contrary, there has been evidence that capital punishment is a motivator for some killers as a means for public martyrdom.
8) There are very serious doubts about the quality of some of the public defenders used by accused in capital cases. There are few wealthy persons that are on death row.
9) It brings out the worst in some. Seeing video of capital punishment supporters at execution sites makes me think that some of the more fanatic persons would have fit it quite well as joyful and self-righteous spectators at witch burnings. Perhaps there were such spectators for the Crucifixion.
10) It desensitizes society and so moves some people further down on the slippery slope: abortion, euthanasia, infanticide, and involuntary sterilization/eugenics.
The above are secular reasons. For me, the most important reason is religious. The Catholic Church teaches the importance of "wearing the seamless garment" and this means having a consistent and non-hypocritical approach to moral issues. The catechism teaches respect for life and this means for the lives of all persons no matter how vile some of them may be. It is not an easy doctrine, but it is a necessary one. The catechism does acknowledge the responsibility of the State to protect its citizens from those who would do harm, but it also clearly states that capital punishment is a last resort and can only be used when there is absolutely no alternative available. Pope John Paul II is a strong opponent of capital punishment and has stated that it has no place in any civilized nation, and I agree.
A quote from J. R. R. Tolkien's _Lord of the Rings_:
"Does he deserve death? I daresay he does. Yet many who die deserve life. Can you give it to them? Then do not be so quick to condemn others, as even the Wise cannot foresee all ends."