The death sentence is a rather final method of removing those genes from the common pool in order to avoid propagating the behavior.
Half arsed eugenics for the criminally insane now? Can we not?
Anyway, because I haven't really made any declarative statements on this, my general view;
The death penalty is an abomination that should not be allowed in any nation for any reason. General reasons for this;
1) Personal moral belief that killing another person is never excusable and that a state doing so makes all those in the state complicit with such an act, even against their will.
2) The state will always have the power to kill people, but this should be so absolutely limited as to be an extreme matter when it does happen. Meaning it should only be an absolute last resort in cases where there is an immediate threat that must be dealt with for the safety of others. That it will happen in policing and military situations is unavoidable, but should still be subject to review and enquiry and always viewed as a failing. Giving the state any power to kill outside those conditions, or viewing any execution as an acceptable outcome the state actively worked
towards, is extreme and dangerous.
3) The death penalty is irreversible, absolutely closing the door to any future revelations of innocence or potential rehabilitation. And I do not believe that rehabilitation is ever absolutely impossible. That doesn't mean it will happen in every case eventually, but rather than leaving the door open for remorse, education and eventual change always makes sense.
4) Speaking here of the US in particular; it is grotesquely expensive while still being disgustingly inefficient. That it is recognised so widely as inhumane and unacceptable makes it incredibly hard for the US to execute people, meaning that appeals drag on for decades and efficient methods of execution are blocked. Doctors in the US are blocked from assisting with executions by at least one major medical body, while the drugs used have been the subject of trade blocks by countries and companies opposed to their use.
This has predictable outcomes. Combine that with the special treatment given to death row prisoners and you have high costs combined with inhumane executions.
5) And still looking at the US, you still have all this happening in a distorted legal system, where prosecutors and politicians are motivated to push for the death penalty even when doubt is cast on a conviction. Combine that with systematic inequalities in the American justice system and you have a disproportionate amount of
black Americans on death row. Being black is a stronger predictor for the death penalty being used than
legal aggravating factors such as a murder committed alongside another felony. While you could say that these are problems with the system and not the death penalty, leaving such a powerful tool in the hands of such a flawed system seems absolutely backwards to me. That would be an argument for at least a moratorium on the death penalty till the other flaws in the system are worked out.
6) It does not work to deter future crimes, is an inefficient method of removing someone from the population (due to economic and moral costs to society being far higher than life imprisonment) and obviously doesn't work as a method of rehabilitation, so doesn't serve any valid purpose of punishment.
Further to that last point, my general view of legal punishment is that it should be tailored to achieve the greatest good for society at the lowest cost to that same society. I would strongly argue this means you can only take into account deterrence, compensation, safety (removing a direct threat) and rehabilitation. Punishment for punishment's sake should not be allowed, except insofar as it has a deterrence or rehabilitative function. Any punishment which could be more efficiently replaced with one with lower social costs should be. So generally I believe that even imprisonment should be used lightly, with direct rehabilitative functions used as a first priority, combined with compensative labour or fines. Imprisonment should generally only be used to remove threats, as an extreme rehabilitation tool or as extreme deterrence method when financial or other factors won't make the impact.