Among "western" countries the United States is alone in its commitment to the death penalty. Australia, New Zealand, the UK and all the EU states, as well as most South America states abandoned the practice decades ago:
https://en.wikipedia.org/wiki/Capital_punishment
I think it's been clear for a long period of time that the death penalty does not serve any logical purpose.
Check out this Australian Institute of Criminology paper from 1987 - 10 years after last person sentenced to death was hanged in Australia. It basically shoots out of the water any potential argument for the death penalty reconcilable with what are generally considered the principals and goals of criminal punishment - it's
not a deterrence to violent crime, it doesn't prevent recidivism because murder doesn't have high rates of recidivism to begin with, it's certainly not rehabilitative and there is a terrible risk of executing innocent people. The only arguments that really hold water centre around retribution and proportionality because there founded on abstract moral and ethical grounds with little grounding in the practical.
As for whether it's a "cruel and unusual" punishment, the question is more vexed. Cruel and unusual is a legal concept derived from the 8th amendment to the US constitution. Thus far the US Supreme Court has never ruled that capital punishment in and of itself is impermissible as cruel and unusual punishment. All they've done so far is impose certain limits on who can be executed, when and in what method:
https://www.law.cornell.edu/wex/death_penalty
The majority opinion in the controlling case. Gregg v Georgia rejected the notion that the death penalty was cruel stating:
[T]he punishment of death is not cruel within the meaning of that word as used in the Constitution. It implies there something inhuman and barbarous, something more than the mere extinguishment of life.
Again, in Louisiana ex rel. Francis v. Resweber, 329 U.S. at 464, the Court remarked:
The cruelty against which the Constitution protects a convicted man is cruelty inherent in the method of punishment, not the necessary suffering involved in any method employed to extinguish life humanely.
And in Trop v. Dulles, 356 U.S. at 99, Mr. Chief Justice Warren, for himself and three other Justices, wrote:
Whatever the arguments may be against capital punishment, both on moral grounds and in terms of accomplishing the purposes of punishment . . . , the death penalty has been employed throughout our history, and, in a day when it is still widely accepted, it cannot be said to violate the constitutional concept of cruelty.
The Court established two conditions that had to be satisfied by State Death Penalty statutes:
- First, the scheme must provide objective criteria to direct and limit the death sentencing discretion. The objectiveness of these criteria must in turn be ensured by appellate review of all death sentences.
- Second, the scheme must allow the sentencer (whether judge or jury) to take into account the character and record of an individual defendant
In summary, the concerns expressed in Furman that the penalty of death not be imposed in an arbitrary or capricious manner can be met by a carefully drafted statute that ensures that the sentencing authority is given adequate information and guidance. As a general proposition, these concerns are best met by a system that provides for a bifurcated proceeding at which the sentencing authority is apprised of the information relevant to the imposition of sentence and provided with standards to guide its use of the information.
https://www.law.cornell.edu/supremecourt/text/428/153
https://en.wikipedia.org/wiki/Gregg_v._Georgia
The UN Human Rights Committee has reached similar conclusions in it's interpretation of Article 36 of the ICCPR. Despite favouring Abolition, it stated that:
While it follows from article 6 (2) to (6) that States parties are not obliged to abolish the death penalty totally they are obliged to limit its use and, in particular, to abolish it for other than the “most serious crimes”. Accordingly, they ought to consider reviewing their criminal laws in this light and, in any event, are obliged to restrict the application of the death penalty to the “most serious crimes”. The article also refers generally to abolition in terms which strongly suggest (paras. 2 (2) and (6)) that abolition is desirable.
http://www.refworld.org/docid/45388400a.html
So it seems that in spite of the fact the death penalty isn't logically defensible, judicial bodies are reluctant to rule decisively against it.
But this is not universal. The South African Constitutional Court in S v Makwanyane did rule the death penalty was constitionally impermissible as it violated rights against Cruel, inhuman and degrading punishment:
http://www.saflii.org/za/cases/ZACC/1995/3.html
The carrying out of the death sentence destroys life, which is protected without reservation under section 9 of our Constitution, it annihilates human dignity which is protected under section 10, elements of arbitrariness are present in its enforcement and it is irremediable. Taking these factors into account, as well as the assumption that I have made in regard to public opinion in South Africa, and giving the words of section 11(2) the broader meaning to which they are entitled at this stage of the enquiry, rather than a narrow meaning,122 I am satisfied that in the context of our Constitution the death penalty is indeed a cruel, inhuman and degrading punishment