Forgiving, when it comes to something as grave and difficult as forgiving someone who has brutally raped and killed your loved one, is obviously very difficult and painful . Nonetheless, if we have the wisdom and insight to place this act in the broad context of the working of the ineluctable Law of Cause and Effect(karma), and our innumerable past lives, our present life and our innumerable future lives, we will be able to correctly view this grossly atrocious act as being the ripening of karma of the victim, however close that victim is to us. "What goes out from us will come back to us". That's how karma works. Karma has returned to the earlier perpetrator of a similar act done to the current perpetrator.
Hence, forgiveness is very necessary to stop the vicious cycle of Cause and Effect which will continue to operate if we continue to give to way to our deluded need for retaliation.
Forgiveness starts the road to inner healing. Inner healing is the healing of the inner disease of hate, anger and the like, which are the root cause of suffering. If one can let go and forgive, one has already begun to show compassion. One grows inner peace. Inner peace and compassion are counters and antidotes to our poisons of hate and anger.However, it is the wisdom of realizing the law of Cause and Effect(Karma) that directly serves as an antidote to our ignorance and delusions, and which sets us on the path of forgiveness and breaking the vicious cycle and downward spiral of retaliation and revenge and suffering.