No one has to forgive but it is probably best to purge the negativity from your system and move on. Think of it this way - if you hold onto the hurt and keep revisiting it then by emotional association they are hurting you over and over again.
By releasing that you are saying "I forgive you and I forbid you from hurting me any more."
Because of my children I have forgiven my ex, moved forward with my life and honey, let me tell you... I am so much better off for it! In hindsight, forgiveness is a blessing and releasing my ex was the BEST 180 pounds I've ever lost. :)