Eugene Volokh is a leading conservative constitutional scholar and a darling of the right-wing Federalist Society. I'm going to
let his words speak for themselves:
[T]hough for many instances I would prefer less painful forms of execution, I am especially pleased that the killing -- and, yes, I am happy to call it a killing, a perfectly proper term for a perfectly proper act -- was a slow throttling, and was preceded by a flogging. The one thing that troubles me (besides the fact that the murderer could only be killed once) is that the accomplice was sentenced to only 15 years in prison, but perhaps there's a good explanation.
I am being perfectly serious, by the way. I like civilization, but some forms of savagery deserve to be met not just with cold, bloodless justice but with the deliberate infliction of pain, with cruel vengeance rather than with supposed humaneness or squeamishness. I think it slights the burning injustice of the murders, and the pain of the families, to react in any other way.
And, yes, I know this aligns me in this instance with the Iranian government -- but even a stopped clock is right twice a day, and in this instance the Iranians are quite correct.
UPDATE: I should mention that such a punishment would probably violate the Cruel and Unusual Punishment Clause. I'm not an expert on the history of the clause, but my point is that the punishment is proper because it's cruel (i.e., because it involves the deliberate infliction of pain as part of the punishment), so it may well be unconstitutional. I would therefore endorse amending the Cruel and Unusual Punishment Clause to expressly exclude punishment for some sorts of mass murders.
Naturally, I don't expect this to happen any time soon; my point is about what should be the rule, not about what is the rule, or even what is the constitutionally permissible rule. I think the Bill of Rights is generally a great idea, but I don't think it's holy writ handed down from on high. Certain amendments to it may well be proper, though again I freely acknowledge that they'd be highly unlikely.
In any event, there's nothing unconstitutional about letting victims' relatives participate in the execution; it's only the use of cruel means that would require an amendment.