Don't we like to think of Jesus as meek and mild, sweet and kind? He is that, except when he's tough and sharp and setting boundaries against our self-destructive impulses. Unforgiveness, which is anger stored beyond it's expiration date, will make us sick. The resentment and bitterness rots us. Jesus is just telling us the truth–if we choose an attitude of unforgiveness, we will pay a price–a high price!
If we are in emotional turmoil, we do well to check our levels of anger/resentment/bitterness. These emotions, I'm convinced, are the torturer's den Jesus is describing. He wants to save us from ourselves when he warns us against unforgiveness. Satan whispers in our ear, "You're right, you should be furious." Jesus stands on our other side, shouting, "Let me handle the injustice. You'll be free. Let it go!"
For some of us, though, the attraction to being right is literally stronger than our desire to be free. We would rather be right than healed. Yikes. What enormous power our choices wield.