How can you forgive (and not only forget) evildoers without any remorse?
The prerequisite for forgiveness is actually completely straightforward and easy to comprehend:
Understand and accept the humanity of the perpetrator.
Most people are convinced that there is a fundamental difference between themselves and those that commit heineous crimes. Once you understand that the difference is at best gradual; once you face your own propensity for cruelty and violence; once you understand how an unfortunate combination of hereditary and environmental factors have lead to the deed -- you have laid the foundation upon which you may forgive the individual guilt.
This is really a broad topic, but think how the Milgram experiments showed how almost anyone could be talked into torturing another even without any valid reason. Now add to that everything that can go wrong in a person's life and it becomes quite understandable how someone can break and turn into a &quot;monster&quot;. I am often surprised there is actually so little major crime!
For some victims it is helpful to learn what made the person that abused them who they are.
The second part is the wish to forgive. For that you need to understand the futility and destructiveness of revenge. Revenge does not bring back a killed loved one; it does not undo the experience of torture; all it does is turn you into the kind of person you despise: a person that tortures and kills. Successful revenge does not lead to happiness. Where forgiveness can cause a feeling of enlightenment and release, revenge fixes the state of loss and trauma and adds to it your own guilt.
Therapy will grant you a space and time for sorrow and rage and then attempt to help you let go and realize that a good and happy life are still possible if you turn towards it and allow it to happen.
This is usually not something that can be achieved by reading books. If you suffer from trauma you will profit from seeing a qualified therapist.