From a comment on a Radley Balko post about Troy Davis:

…the state can’t be trusted to sort the innocent from the guilty with the 100% accuracy necessary for executions to be morally defensible, even if death is a theoretically just punishment…

From what I read everyday it’s abundantly clear that this is true. The criminal justice system has corrupt prosecutors and judges, drastically inadequate public defender resources, corrupt forensic scientists, and plenty of bogus testimony and pseudoscience sold to juries as proof. The latest news could change everything: DNA evidence might be forgeable. If this can be done, a corrupt law enforcement individual will mostly certainly manufacture DNA evidence in the future.

From the ACLU’s Death Penalty Q&A:

Since 1973, 123 people in 25 states have been released from death row because they were not guilty. In addition, seven people have been executed even though they were probably innocent. A study published in the Stanford Law Review documents 350 capital convictions in this century, in which it was later proven that the convict had not committed the crime. Of those, 25 convicts were executed while others spent decades of their lives in prison. Fifty-five of the 350 cases took place in the 1970s, and another 20 of them between 1980 and 1985.

…Who gets the death penalty is largely determined, not by the severity of the crime, but by: the race, sex, and economic class of the prisoner and victim; geography — some states have the death penalty, others do not, within the states that do some counties employ it with great frequency and others do not; the quality of defense counsel and vagaries in the legal process.

…Poor people are also far more likely to be death sentenced than those who can afford the high costs of private investigators, psychiatrists, and expert criminal lawyers. Indeed, capital punishment is “a privilege of the poor,” said Clinton Duffy, former warden at California’s San Quentin Prison. Some observers have pointed out that the term “capital punishment” is ironic because “only those without capital get the punishment.”

…study after study has found serious racial disparities in the charging, sentencing and imposition of the death penalty. People who kill whites are far more likely to receive a death sentence than those whose victims were not white, and blacks who kill whites have the greatest chance of receiving a death sentence. … Minorities are death-sentenced disproportionate to their numbers in the population. This is not primarily because minorities commit more murders, but because they are more often sentenced to death when they do.

Setting aside all moral and emotional arguments, by using the few cases where there is indisputable proof of guilt to justify capital punishment, we guarantee it will be abused to execute many more innocent persons.