First of all lets look at what David said to Saul. When Saul leaves the cave, David follows him out to talk to him, tom garment in hand. He wants to prove both his innocence and his integrity. More than revenge, David desires the Lord's will to be done. Now lets look at what Saul said to David.
Revenge is as old as human nature.
And, while not all vengeance creates a tragedy as bitter as Cain and Abel's, it all makes a travesty of God's plan for our lives. If you're plotting revenge, however small and insignificant you may think it to be, surrender it now. Release your grip on your get-even scheme, and place the control in God's hands. Since humanity is corrupt, expect to be mistreated. The same human nature that beat in Saul's heart beats in all our hearts.