On the one response nobody predicts
“Forgiving is the only reaction which does not merely re-act, but acts anew and unexpectedly.”
Arendt is not making a moral case. She is making a mechanical one: everything else that follows an injury is determined by it, including the retaliation that feels like a decision.
Forgiveness interests her because it is the one response the original act cannot predict. That is also why it is difficult — it means giving up the satisfaction of a reply that fits, and doing something the other person has not earned and cannot anticipate.