Why I have problems with the death penalty

While I support the theory of the death penalty - that there are certain people who cannot be tolerated in a free society, and certain crimes so heinous that death is an appropriate response - my considerable unease about the practical implementation of the death penalty is perfectly expressed by this case: the jailing of Texan Michael Morton for the murder of his wife where, after 25 years in jail, his legal team finally managed to discover proof that he was innocent:

So it was a sudden reversal of fortune for [prosecutor] Anderson when, eight days after Michael's release, the CCA overturned Michael's conviction on grounds of actual innocence. The ruling meant that Anderson had secured a guilty verdict against an indisputably innocent man. Yet whether he, or anyone else involved in the case, would ever be held accountable for the wrongful conviction remained an open question.

It seems that, in their zeal to prosecute Morton, the prosecution team had failed to disclose key evidence that would have pointed Morton's defence team at a lead which contradicted the claim that Morton was the killer. Now the prosecution's job is to present the state's case in the best possible light, but that doesn't (or at least shouldn't) extend to hiding exculpatory evidence from the defence.

Morton was lucky not to get the death penalty for the crime of which he was falsely accused. If he has (finally!) been exonerated, due to a tireless defence team, how many more people sitting on Death Row (or buried in an grave) have been victims of the same practices but less fortunate?