The reaction of the people I have talked to was that the Sheldons must have been real Christians. And more than a little nuts. Several felt it was almost obscene to forgive their girls' killer. What's wrong with forgiveness. It is unnatural.
Dogs eat dogs.
If forgiveness was natural, it would be easier. B. It is unfair. Our sense of justice yearns to be vindicated. Forgiving may let a bad person off the hook. They may do it again.