Yes, but it’s debatable about what’s wrong and what’s right.
I believe it’s generally accepted by most of society that people found guilty of the crime of first degree murder should be punished for the crime they committed. It would be nice if restitution was involved, too, but I don’t think that occurs very often in homicide cases.
Does anyone believe no punishment should be involved with first degree murder? If so, then it follows that if, hypothetically, the recidivism and deterrence rates were both zero, murderers should simply not be prosecuted at all. But, that doesn’t feel right, does it? There is the issue of justice. Even if punishment resulted in no net gain for society, we still believe that willful first degree (no mental disease involved) crimes against society should be punished—at least I hope we all believe that.
Then, it becomes a matter of what is the appropriate punishment to fit the crime? We could, of course, punish first degree murderers by smacking them 20 times with a wet noodle, but that doesn’t feel right, either. Somehow, we know at our cores, that punishments should be somewhat equal in severity to the crime committed. Well, an “eye for an eye” punishment is certainly the easiest way to obtain equality. However, in most cases, I think the majority of us agree that “an eye for an eye” is not the best punishment for the state to be divvying out. Raping a rapist doesn’t sit well with me for some reason. Although, I would bet heavily that many convicted rapists would accept the punishment of being raped by a state appointed rapecutionist as opposed to living the rest of their lives in a penitentiary.
But, I think an “eye for an eye” punishment may be appropriate for some situations, including the heinous crime of first degree murder. As mentioned, it’s certainly equivalent to the committed crime. But, is it wrong, or immoral for the state to kill in certain circumstances? I don’t think so. On what grounds, religion? Maybe God was just using shorthand on Moses’s tablets—should have been “thou shalt not kill unless it’s as punishment for 1st degree murder.” Anyway, I believe the great majority of Dopers are areligious, so this should not even be an issue. Is killing in any circumstance a universal immorality?
Why else would it be immoral for the state to kill convicted murderers? Most of us don’t have a problem with soldiers killing enemies of the nation during times of war. How is that different than killing enemies of the state during times of murder? Frankly, I find killing enemy soldiers more distasteful than killing 1st degree murderers.
Of course the alternative punishment for 1st degree murder cases heinous enough to garner capital punishment, is life in prison without the chance of parole. That is arguably on equal footing, punishment-wise, to capital punishment. But, is it really better and is it really more moral? In the real world, I do think it may be better, but only insofar as it allows a reprieve for those rare cases of wrongful prosecution. However, the OP’s magic hypothesis assures against wrongful prosecution, so this is not relevant.
So, as per the OP scenario, you really have only two possibilities: 100% guilty murders being put to death by the state vs. 100% guilty murderers spending the rest of their lives in prison without hope of parole. Why do some of you think the second choice is more moral than the first?
I believe most of you will agree that spending life in prison without hope of parole is a miserable existence, correct? In fact, if we were put an innocent person in prison for life, that could be considered a form of torture. We put our pets to sleep in order to put them out of their misery and are considered humane for doing so. How is this different than putting inmates to sleep and putting them out of their miserable existence? In fact, it can be argued that putting convicted murderers to sleep may be even more humane than putting pets to sleep: not only are you relieving them the misery of life-long incarceration, you may also be relieving some of them the mental anguish of life-long guilt for the misery they caused others (at least the ones with a conscience).
All in all, I think capital punishment and life in prison without the chance of parole are pretty equivalent forms of punishment and either is appropriate for the crime of 1st degree murder. With the chance of wrongful prosecution, I give the nod to life in prison; with no chance of wrongful prosecution, I give the nod to capital punishment. With inclusion of my magical hypothetical of 25% deterrence, I give the nod heavily to capital punishment.