Then David set out and came to the place where Saul had encamped; and David saw the place where Saul lay, with Abner son of Ner, the commander of his army.  Saul was lying within the encampment, while the army was encamped around him. 

Then David said to Ahimelech the Hittite, and to Joab's brother Abishai son of Zeruiah, "Who will go down with me into the camp to Saul?"  Abishai said, "I will go down with you."  So David and Abishai went to the army by night; there Saul lay sleeping within the encampment, with his spear stuck in the ground at his head; and Abner and the army lay around him.  Abishai said to David, "God has given your enemy into your hand today; now therefore let me pin him to the ground with one stroke of the spear; I will not strike him twice."  But David said to Abishai, "Do not destroy him; for who can raise his hand against the Lord's anointed, and be guiltless?"  David said, "As the Lord lives, the Lord will strike him down; or his day will come to die; or he will go down into battle and perish.  The Lord forbid that I should raise my hand against the Lord's anointed; but now take the spear that is at his head, and the water jar, and let us go."

"The Lord rewards everyone for his righteousness and his faithfulness; for the Lord gave you into my hand today, but I would not raise my hand against the Lord's anointed."

The Lord cast a deep sleep on Saul and all of his men when David discovered their camp.  Yahweh delivered Saul into David's hands but it was not for the purpose of vengeance, as Abishai supposed, but mercy.  The Lord may have been testing David.  If so, he passed.  But it is certain that Yahweh offered to David a great work of mercy to perform.  "As today I set great value by your life, so may Yahweh set great value by my life."  "Blessed are the merciful: they shall have mercy shown them." (Matthew 5.7)

This day it may well be that someone you dislike, or that dislikes you, is given over into your hands.  Perhaps you hear some gossip about the person or learn some weakness about them - something that will give you an advantage over them.  The Lord is perhaps testing you; he is most certainly offering you a work of mercy to perform.  Show mercy, that God in turn show you mercy!  Keep your act of mercy secret and hidden, known only to God.  Do not swell up with pride that you acted mercifully, say rather "I am a useless servant, having done only what is required of me." (Luke 17.10)  Further, these acts are given us by God to perform, we do not create them. (Ephesians 2.10)