Both the Old Testsment and the New Testament address the danger of unforgiveness.  There are those who really believe that unforgiveness is the right way to address an injustice or personal wrong.  This may seem the right approach, but it is a deadly approach.  Lack of forgiveness is a spiritual trap.  It saps the life of the one who will not forgive.  Unforgiveness will leave one empty and spiritually barren.  It is imperative that one forgives an offense because otherwise one will find himself in the same prison with the offending party!