You need to blur the line in the perps mind (for crimes like this) as to whether or not they have actually gotten to hell yet. IMO, cruel and unusual is only such if it’s arbitrarily and capriciously administered and to the obvious entertainment of the administrator.
In sexual harassment law, much of what constitutes harassment is actually defined by the perception of the victim. i.e., if you tell a joke with no offense intended, but somebody takes it the wrong way, you are guilty of harassment.
Perhaps the same standards should be applied to the definition of "cruel and unusual." In as much as these two fellows felt that vivisection was a perfectly appropriate avenue for their victim, they would be hard pressed to argue that it was cruel and unusual once they've been sentenced to the same.