Let me start out by stating that, in principle, I am not opposed to the capital punishment for people who have consistently and repeatedly demonstrated by violent and murderous actions over time that they have no regard for human life. I’m not necessarily pro-capital punishment in such circumstances (because I think there are probably better ways of dealing with the situation) but I understand that putting such violent psychopaths to death is probably a legitimate response, given that their actions demonstrate that they don’t themselves value life in any way. Such a violent psychopath would probably not object to their being put to death either.

In practice, however, I am vehemently opposed to capital punishment. The current so-called justice system is fatally flawed in at least two areas when it comes to capital punishment; one of which makes the practice an absolute moral abomination. The other is merely a matter of pragmatism.
<!–more–>The two facts that prohibit the use of the death penalty under the current system are:
1 – Innocent people are sometimes sentenced to death and put to death.
2 – There is good evidence that putting people to death is at least as expensive as imprisoning them for life without parole.
The first fact – that innocent people are frequently sentenced to death – is axiomatic at this point. The Innocence Project has documented 280 post-conviction DNA exonerations, including 17 exonerations of people sentenced to death (who, by the way, were in prison for a combined total of 209 years)
There have been 23 exonerations of death-row inmates in Florida, the most of any state.
Wikipedia lists 139 people who have been exonerated after being sentenced to death. The same list is available at the Death Penalty Information Center.
The Capital Defense Network lists 26 people through 2002 who were exonerated just before being put to death, including a 1999 case where a man was exonerated just nine hours before being killed.
I know what you’re thinking now. You’re thinking, “Isn’t this the way the system is supposed to work? Aren’t these post-conviction exonerations proof that the innocent are being weeded out before they are put to death?”
Unfortunately this is not the case. Beside the fact that the exonerated have lost years of their lives, the Death Penalty Information Center lists nine people who were put to death since 1989 who were exonerated after being executed. On the list is Cameron Todd Willingham, a man who was recently exonerated after being convicted of committing arson to his own home, killing his 3 children. He was put to death in 2004. After questions were raised about his guilt, Texas established a commission to investigate. Craig Beyer, a fire scientist on the commission, found that

investigators in the Willingham case had no scientific basis for claiming that the fire was arson, ignored evidence that contradicted their theory, had no comprehension of flashover and fire dynamics, relied on discredited folklore, and failed to eliminate potential accidental or alternative causes of the fire. He said that Vasquez’s approach seemed to deny “rational reasoning” and was more “characteristic of mystics or psychics.”

Discover Magazine recently published an article by Douglas Starr detailing the dismal state of fire investigation which relies on old wives’ tales and speculation rather than rigorous scientific principles.

…it is becoming clear that over the past several decades, dozens, perhaps hundreds, of people have been convicted of arson based on scant research and misguided beliefs. Many of those people are still in jail, hoping that someone will take up their cause.

“I shudder to think how many wrongful convictions there are,” says Richard Roby, president and technical director of Combustion Science and Engineering, a fire-protection engineering firm based in Columbia, Maryland. Roby has testified for several men charged with arson. One, named Michael Ledford, could not have been at the scene when the fire that killed his son was allegedly set, according to Roby’s calculations, yet he is now serving a 50-year sentence. “It’s amazing to think how long it takes for basic science to be accepted,” Roby says. “I lose sleep over this every week.”

With some 7626 people sentenced to death since 1977, 139 post-conviction exonerations represent a 2% failure rate. This number is likely low because

Courts do not generally entertain claims of innocence when the defendant is dead. Defense attorneys move on to other cases where clients’ lives can still be saved.

There is no way to tell exactly how many innocent people have been put to death in the United States, but the actual ratio of innocent to guilty people sentenced to death is probably much higher than the known ratio of 1 in 50.
Moving from capital punishment to life in prison without parole is a better solution because one can be paid off by the government for false imprisonment after any number of years. There is simply no way to bring back an innocent person from the dead. Putting someone to death is final and absolute. There is no going back.
The second fact that makes capital punishment indefensible is that, in the current system, putting someone to death is probably more expensive than imprisoning them for life. Before continuing, please search Google for “death penalty cost” to see the wealth of information on the topic.
Estimates vary, and good data is hard to come by with regard to the financial cost of the death penalty, so there is not yet a firm consensus about this issue. However, the evidence is beginning to show a clear trend: under the current system, sentencing people to death and carrying out the sentence is more expensive than the sentence of life without parole.
Most of the claims that the death penalty is cheaper than life without parole contain the small caveat that the death penalty COULD BE cheaper if the system was reformed. Even the website ProDeathPenalty.com (which concludes that the putting prisoners to death is cheaper than sentencing them to life without parole) includes the following disclaimer:

NOTE – 10/19/00 – We received a post which located a flaw within our cost evaluation. The reader stated that we should “present value” all the costs of both a life sentence and the death penalty and that, if we do so, a life sentence is cheaper than a death sentence. Using the numbers in our analysis, such is a good point.

One wonders why they declined to reverse their conclusion.
Perhaps the most authoritative study on the issue is a 1993 Duke University study which compared capital cases and life without parole cases and found that compared to life-without parole cases,

The extra cost per execution of prosecuting a case capitally is more than $2.16 million.

That estimate includes all trial and post-trial costs including incarceration and is in comparison to the trial and post-trial costs of for people who were incarcerated for life without parole.
A New York Times editorial sums up some of the data nicely:

…keeping inmates on death row in Florida costs taxpayers $51 million a year more than holding them for life without parole. North Carolina has put 43 people to death since 1976 at $2.16 million per execution. The eventual cost to taxpayers in Maryland for pursuing capital cases between 1978 and 1999 is estimated to be $186 million for five executions.

Perhaps the most extreme example is California, whose death row costs taxpayers $114 million a year beyond the cost of imprisoning convicts for life. The state has executed 13 people since 1976 for a total of about $250 million per execution…

Given the two facts outlined above, capital punishment must be ended until a means can be found to ensure that:
1 – no innocent people are executed (something which is likely impossible) and
2 – the costs associated with capital punishment are brought below the costs of imprisonment for life without parole.