@gungasnake,
You've expressed the source of my ambivalence concerning the death penalty: Fundamentally, I am not comfortable with the State being able to legally kill its citizens.
Not everyone will agree with your assessment of the injustices committed in the cases you've cited, but there is ample evidence of prosecutorial misconduct in wide range of cases involving a wide range of defendants to support your point.
Whether or not it actually serves as a deterrent, has been buried under numerous conflicting studies, but, in my opinion, that's the least of the reasons to support the death penalty.
The cost issue is something of a red herring as without innumerable appeals, it's cheaper to execute prisoners rather than imprison them for life, and, in any case, if capital punishment is a necessary element of bringing justice to certain criminals, we should spend the money on it.
I don't object to capital punishment or moral grounds either. If you take someone's life you are robbing them of everything they have or ever can have. The extent of the crime is even more pronounced if you do not subscribe to a belief system that promises a hereafter. Those guilty of such crimes have, in my opinion, forfeited their own right to life. Retribution is, in and of itself, an appropriate goal.
However, as you have noted, we can't expect or trust the State to unfailingly pursue justice in each and every criminal case. Even if we were comfortable with the State having the legal right to kill its citizens who are guilty of crimes we agree deserve the death penalty, we cannot rely upon it to do so without error, unintentional or intentional.
If the State is going to wield God-like power over its citizens, then it needs to have God-like omniscience, which of course it does not. Better for all of us if we sacrifice some measure of retribution for the necessary imposition of limits placed on the only entity which we can even consider for this power.
Allowing the victims of heinous crimes to deliver justice to the perpetrators may seem like perfect retribution and is emotionally very satisfying, but aside from the fact that the victims will err at least as often as the State, members of a society that has the Rule of Law as its foundation, must surrender the dispensing of justice and retribution to the larger society.
Of course people can always choose to operate outside of the laws of their society, but they can't choose to not be subject to them. Vigilantes make for good comic book and movie heroes, but destructive members of society.