My arguments that the death penalty is cruel and unusual punishment are essentially those of Breyer in his dissent in
Glossip v. Gross. See posts #1 and #2 here:
Is the Death Penalty Constitutional?
Compassion for murderers is not a component of any argument I made or that Justice Breyer made (I strongly encourage you to read those two posts).
To briefly summarize what I consider the strongest and most important arguments of Justice of Breyer, I noted first and foremost that the death penalty of unconstitutional because it is cruel and unusual. As I noted and quoted at that the above thread:
Quoting
Atkins v. Virginia (which quotes
Enmund v. Florida), he notes that “if the the death penalty does not fulfill the goals of deterrence or retribution, ‘it is nothing more than the purposeless and needless imposition of pain and suffering and hence an unconstitutional punishment.’” But the death penalty can’t fulfill the goal of deterrence because, as demonstrated by unequivocal evidence that Breyer cites, it actually is an
unusual punishment (and thereby serving one criterion of its unconstitutionality):
. . . if we look to States, in more than 60% there is effectively no death penalty, in an additional 18% an execution is rare and unusual, and 6%, i.e., three States, account for 80% of all executions. If we look to population, about 66% of the Nation lives in a State that has not carried out an execution in the last three years. And if we look to counties, in 86% there is effectively no death penalty. It seems fair to say that it is now unusual to find capital punishment in the United States, at least when we consider the Nation as a whole. See Furman, 408 U. S., at 311 (1972) (White, J., concurring) (executions could be so infrequently carried out that they “would cease to be a credible deterrent or measurably to contribute to any other end of punishment in the criminal justice system . . . when imposition of the penalty reaches a certain degree of infrequency, it would be very doubtful that any existing general need for retribution would be measurably satisfied”).
Punishment by death is also unusual on the international scene, at least among developed countries:
Capital punishment by country - Wikipedia
One simply cannot argue that the death penalty is anything but an unusual punishment, rarely practiced in modern societies.
Further, imposing the death penalty is a gross imposition on society due to its cost. Again quoting a couple of facts noted in the above thread:
One of the primary reasons that so many states have either repealed or abandoned their death penalty statutes is due to the stunning cost. A 2008 report by a California Commission found that the death penalty costs the state $137 million per year whereas comparable life sentences without parole would cost $11.5 million per year. A 2000 investigative report in the Palm Beach Post calculated that each execution in Florida cost an additional $23 million above a sentence of life without parole.
Justice Breyer argues the cruelty prong of the 8th Amendment restriction primarily on three bases, the first 2 of which are:
(1) Sentences imposing the death penalty are unreliable in that a significant portion of people on death row or are put to death are found to be actually innocent. A much larger portion of people sentenced to death are wrongfully convicted.
(2) The death penalty is arbitrarily imposed. The most heinous crimes often do not result in the death penalty, and there is certainly a racial (racist) component where the death penalty is imposed.