To me, forgiveness is a culturally bound concept - whose definition is contextual rather than neurological.
It is also bound to the idea of "pardoning." But pardon is something that at least here has a stronger association with the justice system (e.g., the governor granted a pardon...) Perhaps when someone who has committed a criminal offense against a person is finally punished societally as in prosecution and sentencing, "forgiveness" is more easily attained.
If criminal justice is then a form of "revenge" then it's not enough to make the system work. One presumes that criminal justice involves more than the revenge factor. Therefore, one could "forgive" and not "pardon."
Another factor that ties into forgiveness is the extent to which a perpetrator is so horrified by their own actions that they would never commit such an act again. Forgiveness then is also tied into the reaction of the perpetrator.
It is easier to "forgive" someone who receives appropriate societal punishment for their crimes and it is easier to "forgive" someone who is thoroughly repentant.
That being said, there are some actions, some crimes and some cruel human experiments that should not be forgiven and most certainly never pardoned.