This can be one of the hardest things to do. Especially, if one feels a level of hurt that is deeper than mere physical pain. Holding on to resentment harms you more than others.
To forgive can feel like they are getting away with something. In reality, the strength it takes to let go and show mercy is tremendously powerful. No matter what has been done to you, when you show someone who has hurt you absolution, it lets them know you are the bigger person.
This permeates on such a deep level that the guilty (or who you consider guilty) will automatically be reduced not only in stature but in esteem.
No one likes to be wrong and offering forgiveness especially to someone who, stubbornly, does not seek it, will force them to reevaluate their actions. If it makes them change future behaviour that is a good thing. If not, believe in karma, the bad will catch up to the individual eventually.
People that do things requiring clemency need more love than normal. Remember ‘to forgive is divine’ and we all can use that at some point in our lives.