A Matter of Trust
"Now it came about when Saul returned from pursuing the Philistines,
he was told, saying, 'Behold, David is in the wilderness of Engedi.' 
Then Saul took three thousand chosen men from all Israel, and went to seek
David and his men in front of the Rocks of the Wild Goats.  And he came
to the sheepfolds on the way, where there was a cave; and Saul went in
to relieve himself. Now David and his men were sitting in the inner recesses
of the cave.  And the men of David said to him, 'Behold, this is the
day of which the Lord said to you, 'Behold; I am about to give your enemy
into your hand, and you shall do to him as it seems good to you.''"
When he woke up this morning, David had no idea that he would be facing a life-defining decision, but that's exactly what the day would bring. After returning from chasing their enemy the Philistines, Saul got an intelligence report on the whereabouts of his other enemy-David. The report must have been accurate, because Saul wound up in the very cave where David was hiding. Saul was in a compromising position and David had a grand opportunity to kill him, and frankly, I don't think anyone would have blamed him if he did. Saul hadn't treated David right.
The mistreatment certainly wasn't deserved. David had done nothing except try to serve his king. When he offered to fight Goliath, it wasn't because he was seeking for fame or trying to upstage Saul, it was because the Sprit of the Lord was upon him and he couldn't stand listening to anyone, let alone this uncircumcised Philistine mocking the armies of the Living God. When he refused to wear Saul's armor, it wasn't because David wasn't gracious; it was that he knew he couldn't fight the kind of battle he knew how to fight while wearing it. From every indication, Saul was as happy as everyone else when David was victorious against Goliath. Because of his military prowess, David became a commander in Saul's army and repeatedly led his men to victory against Saul's enemies.
David jumped whenever Saul needed him. Saul had terrible mood swings
and when he was being tormented, David would play the harp to soothe his
soul. Beyond his musical duties, David worked hard to protect Saul and
was willing to lay down his life for him if necessary. David was always
humble in Saul's presence. When Saul offered his oldest daughter in marriage
to David, David refused because he did not count his family worthy for
such an honor. Yet, even with David acting honorably toward Saul, Saul
constantly mistreated David.