In my New Jerusalem Bible Commentary it speaks of this passage being read against a background of the faith.
"But the Spirit of the LORD departed from Saul, and an evil spirit from the LORD troubled him." - I Samuel 16:14
There are two traditions of David in Scripture (if memeory serves). In one, he pops up, as it were, and beats Goliath. In the other David was already a member of Saul's household. This is from that tradition.
16:15-18
And the servants of Saul said to him: Behold now an evil spirit from God troubleth thee. Let our lord give orders, and thy servants who are before thee, will seek out a man skilful in playing on the harp, that when the evil spirit from the Lord is upon thee, he may play with his hand, and thou mayst bear it more easily. And Saul said to his servants: Provide me then some man that can play well, and bring him to me. And one of the servants answering, said: Behold I have seen a son of Isai, the Bethlehemite, a skilful player, and one of great strength, and a man fit for war, and prudent in his words, and a comely person: and the Lord is with him."
We can see that the 'evil spirit' is Saul's anxiety, uncertainty, etc (not an evil spirit in the sense of worshipping a false god, or slaughtering babies, etc.,) which God does not calm, therefore must be upon Saul in fulfillment of God's will.
Therefore one might say "The Lord departed from Saul, and left him to his own devices..." but the authors of scripture do not read it that way, but rather that God is working with Saul, even though Saul has already broken his Covenant with God.
The interplay between Saul and David is a very real moment in Jewish history. Saul is a king, annointed by Samuel, but David, also annointed by Samuel, will be king, and in fact Saul is told as much, even before the crown is warm on his head.
The author of the Book of Samuel makes it quite clear that the desire for a king, a demand of the people, was not a providential decision, but one to which God, it appears, assents.
It is, in this sense, an instance in which God would utilise the willfulness of Isreal to being about His plan for their deliverance, despite them-(our)-selves.
David won the confidence of Saul as he won the confidence of the people. For all his faults he is the best of the kings of Israel, and thus becomes the 'type' for the future king foretold. So the authors would discern that God was leading David to Saul, and Saul to David, and also to show that where Saul got into doubt and turmoil, David was serene, or at least calm and composed.
What did Saul prophecy? We do not know, but we can guess it's along the lines of "We're all doomed!" or "This is hopeless!" or "God has abandoned us!" – David turns round and quite calmly says, "No. Trust in God and all shall be well." Saul, infuriated, "who the hell d'you think you are, harpist?" throws his javelin at him ... then posts him to a combat unit in the hope he'll get himself killed. Quite the reverse, David is actually quite a good war commander...
... read on, Dondi! It gets positively Jacobean! If Shakespeare never wrote a play based on this, he should have ... Power, politics, greed, sex, corruption, lies, entrapment, murder, war ... it's all there ...
This is a moment from our Salvation History, and its daggers drawn in the corridors of power!
Thomas