How can you forgive (and not only forget) evildoers without any remorse? Given the mental and physical health benefits of forgiveness, how is it usually approached in psychological treatment? Despite hours of reading, I still don't understand, and can't even pinpoint why not. Please remark the following assumptions which restrict my definition of 'malefactor' to forgive:

1. The malefactor fails to repent or rue in any way, such as Mutsuhiro Watanabe (who tortured Captain Louis Zamperini about whom I write below) or the Holocaust perpetrators.
2. I ask about genuine forgiveness of the malefactor, and not merely forgetfulness.
3. The malefactor recognised his/her actions' turpitude but still wilfully executed them.

This question was motivated by such virtuous paragons as  Ms Eva Kor and Capt Louis Zamperini, the latter of whom advised someone else by citing Mark Twain:

“Forgiveness is the fragrance that the violet sheds on the heel that has crushed it.”

I recognise the futility in simply remaining enraged about past injustices, so I'd argue that one should act to resolve this resentment, such as by apprehending and punishing their captors. This is exactly the argument that has bewitched me but that I'd like to surmount; I want to appreciate: How and why could (even) tortured war veterans and concentration camp survivors decide against any revenge?
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.