Wounds can fester into infections if left untreated. That’s exactly how unforgiveness works. Whatever was done to us pierced our skin, but if we keep prying it open and looking at the wound, it won’t be able to heal… instead, because it is continually exposed to the dirty air, it becomes infected. That infection in the spiritual realm is welcoming to unclean spirits, which fester the wound even more. If something isn’t done, the person ends up facing demonic harassment and torture, and becomes a very bitter and unhappy person.
I have an idea of what you may be saying right now, “This person has NO clue what they’ve done to me! They don’t deserve anything at all! Much less MY forgiveness!!” They certainly don’t deserve your forgiveness, much less God’s… but none of us deserve what Jesus did for us either. Those who killed Jesus didn’t deserve anything at all, but look at what He said just before He died, “Father, forgive them, for they know not what they do!” Look at the deep and rich mercy and love that Jesus has towards us… none of us deserve it! But He loves us for who we are, not because of what we’ve done. He wanted a relationship with us so much that He gave His life for it! When we grasp what Jesus has done for us, it makes it a lot easier to pass that grace along to others. We aren’t letting them off God’s hook, we are merely releasing our souls from bondage that un-forgiveness brings us under. You aren’t forgiving them for their benefit, but for your own good! Your soul, not theirs, is what is being held in bondage because of the feelings you’ve allowed yourself to harbor inside. Why should you allow what they’ve done to continue to bring you under bondage? I wouldn’t! I would let that poison out of my heart… give it to the Lord and seek Him to heal the wounds they have caused.
Forgiving others is sometimes very hard, but it is essential if you want to break out of the bondage that it’s brought you under. Forgiving others opens you up for the Lord to begin healing your soul (inner healing). Since unforgiveness blocks us from receiving God’s forgiveness of our sins (Matthew 6:15), it puts up a wall between us and the source of our healing.