Like many "criminal-friendly" aspects of criminal law, the opposition to the death penalty goes beyond compassion for the guilty; it is motivated by a desire to protect the other innocent from abuse of power, and principled belief that forcibly ending a life is unacceptable because no person is capable and trustworthy to accurately and fairly determining which cases deserve it, and drawing a bright line to separates the kind of criminal people who would kill unnecessarily from the civilized people who would not, and sitting comfortably on our side without engendering doubts.
I might counter-argue that at some point there's no doubt and being permanently removed from the pool of people who are alive might be OK.
What I mean is that if someone is attempting to murder me and I manage to kill them first that it was a justifiable homicide and that I shouldn't be punished in any way. That's in the law and it even makes sense to a great many people even if they're not lawyers, and even if they don't know the finer points of the law. In that situation there is no doubt, or perhaps no reasonable doubt and the law does effectively judge people to be competent to make precisely that decision.
Part of what makes this so difficult is that she actually wielded vast power due to her position. In many ways she was far, far more powerful than a judge or a prosecutor. A judge can't just unilaterally accuse someone of a worse crime just because, nor can a prosecutor reasonably trump charges up too much without getting called out for bad behavior. But it seems that this lady did precisely that, totally perverting justice.
If an accountant cooks the books it might be professional misconduct (and criminal!) and people might lose their jobs and all of that is terrible. But that accountant can't somehow make all the employees of the company go to jail by doing something wrong.
The power that this lady had and the extent to which she abused it is simply breathtaking.
I just want to weigh in that I am really glad that none of those falsely convicted by this faked evidence were sentenced to death. At least those falsely imprisoned by the "evidence" this scum planted can be set free, even though they lost years of their lives.
And that is the argument of why there should not be a death penalty, even for people who you are really sure that they really deserve it. It's a form of humility: To admit that there can be wrongs in the legal system, and so to be very sure not to do anything completely irreversible.
That's why people argue that the death penalty is unacceptable in any case whatsoever: the system has to be able to admit that sometimes it is wrong.
