Monsters. What will their child think when he/she grows up and realizes that Mom and Dad could have killed “it” if the wind had been blowing in a different direction.
Sarah committed the unforgivable crime when she loved her child. They cannot forgive and they cannot forget. It eats at them.
By the time the child grows into an adult, the knowledge won’t surprise them. S/He will already realize how important they weren’t to their parents.