why i can't oppose the death penalty in all cases

[starroute]

52. Then you're left with so few cases it becomes a "cruel and unusual punishment"

That clause is in the Bill of Rights partly because of a case from the middle 1700's where a Frenchman attempted to assassinate the king and was executed by being publicly tortured in a series of nasty ways and then ripped apart by horses. (There's a detailed description somewhere in Dickens' A Tale of Two Cities that traumatized me as a kid.)

This shocked the conscience of an era that considered itself enlightened and led to the conclusion that no matter how appalling or socially destabilizing a crime may be, it doesn't justify barbarities that would otherwise be considered unacceptable.

And that's the principle that your proposal to restrict the death penalty to particularly heinous crimes would seem to violate. If something is morally objectionable enough that you wouldn't want to to it under most circumstances, it doesn't become morally okay just because the circumstances are extreme.

If anything, the person who thinks of the death penalty as barbaric but would apply it in certain selected cases is violating their own moral principles in a way that the person who accepts the death penalty as routine is not. And this is what the blanket prohibition in the Bill of Rights is intended to guard against.