A perpetrator must show remorse and repent for their actions in order to be forgiven. This should not have any bearing on how a Holocaust survivor lives their life. Choosing not to forgive puts the power in the victim's hands because they can decide that they do not have to give peace to the person that harmed them in the past.
In addition, if a Jewish survivor of the Holocaust desires to honor their traditions in forgiveness, they should have the power to do so. In not forgiving the Nazis that show no remorse for their actions, the survivor is abiding by their religious statutes. This is even more empowering than forgiving because the survivor as the full power to exercise their Jewish traditions without fear.