Regarding capital punishment, I can imagine a number of crimes so heinous that they are almost certainly unpardonable. Such black deeds can so shake our faith in humanity that we must instead draw the conclusion that these are the acts of no human, but instead of a monster.
Human rights and protections should only be extended to humans.
For those who universally reject the death penalty, they must be willing to forgive even the worst of acts. I do not believe that many have that capacity, only that they claim to.
Of course, the law is not, and should not be, determined solely on the basis of morality.