1. I'm not sure what you're arguing here. Of course such occurrences are rare, and Capital Punishment should likewise be something only used in extreme cases. To do otherwise would be irresponsible and dangerous.
2. Utilitarianism <-- Since you don't appear to have understood what I meant by utilitarian.
3. It helps when they've written Manifestos and gloat about their accomplishment.
You're right that corruption is the primary factor when considering Capital Punishment though. As in 1, I believe it should only be used in cases where the possibility of false charge is so unlikely as to require a full-blown conspiracy. I'm rather serious about it only being used in the most extreme of cases. Even then I feel it may be less open to abuse if execution was optional on the subject's part, with confirmation only being done alongside a sizable media presence that is made freely available to the public. If the person is informed that they can never be released into society again and chooses to end their life instead of suffering in a cell with no hope for the future, while we apparently keep them alive just so they can suffer longer, then they should have that path open to them. This reduces the potential benefit gained by executing those with no hope but provides yet another hurdle to overcome for potential misuse.
If a government is corrupt enough to ignore all that, then they probably already control the nation to an extent where any laws against Capital Punishment, or the presence of those who oppose it, are irrelevant.
4. If it's not a moral absolute then why are you arguing from that position? You just say that we shouldn't kill each other without paying attention to the circumstances behind the act. If killing is universally wrong then you're arguing that it is a moral absolute.
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If this derail is getting too far away from the incident for the taste of readers then I'll stop at a request.
1&3, feel free to ignore. Responding mostly for the sake of responding.
1. I'm not really arguing anything, beyond the study I linked earlier. Also, cutting costs most likely increase the chances of an innocent person getting executed.
2. Yes, I slightly misunderstood the term. But not too much, and I believe my argument stands...? Maybe I'm just being stupid then. I don't see how killing an unarmed prisoner increases maximum "good", as the link says.
Threat of a death penalty clearly does not work.3. Giving a prisoner an option of suicide I can agree with. Though it raises the question whether it's just an indirect execution and there's not much to choose for the prisoner, but that's irrelevant for now.
4. It's not universally wrong. I see nothing wrong with shooting a suicide bomber, for example, as there usually is no other option.
And yes, the circumstances behind the act are largely irrelevant, when it's beyond the point of "life in prison vs death penalty". It doesn't matter how many the person killed, or how brutally he did it. Only thing that matters is the future. Is this person going to cause more deaths in the near or far future? In this case, if he's locked up for the rest of his life, it's unlikely he'll ever cause another death.
In my view, killing is still generally wrong. It's not some universal value, it's only rational because it makes a better world for everyone. A prisoner sitting in a prison for a life is no threat to anyone, as such it's hypocritical to kill a killer.