Saul was a big man with a little heart! He was head and shoulders above the rest of his countrymen in relation to his stature but he was petty and petulant in his treatment of others, especially David!
There was a natural tension that would exist between these two men! Before we get to 1 Samuel 18 several things had already been divinely decided. Saul had been rejected as King over Israel and David had been anointed as the next king of Israel. One was leaving the throne and one was assuming the throne. It was a long process wherein their paths would cross. They would meet each other; one on the way down, the other on the way up.
Their divinely appointed end was known to each of them respectively. God had told Saul the kingdom was to be taken from him. David had already been anointed the next King of Israel. The question becomes how do each of these men manage the information? They should both accept the will of God with humility.
Saul would continue on the throne as long as God intended with the humble realization that because of his own disobedience God would be preparing another to take his place. He would not take offence at indications that God was grooming another for his place.
David would not press the issue but realize that God would give him the throne in his time. He could wait because he had the promise and anointing of God. He would feel no need to raise a rebellion to supplant the present king. He would in fact honor the present king knowing that one day he would be king.
One man rose to the occasion and acted as he should under trying circumstances. The other man sunk to new lows as he sought to circumvent God's decree. This of course brought them into conflict. David conducting himself with dignity and honor in spite of the ill treatment he received at the hand of Saul. Saul conducting himself with malice and bitterness in spite of the honorable treatment he received at the hand of David.
The growth of the contagion that seized Saul's heart toward David has for years been a point of reference for me to guard my own heart.
1 Samuel 18:9 - Saul eyed David
I actually like to call this "the evil eye". Having heard the women sing Saul has killed his thousands and David his ten thousands had served to remind Saul of his divinely decreed decline. David suddenly became suspect. This in spite of the fact that David had been nothing but faithful to Saul and Israel. Yet, his elevated prominence in the nation caused Saul to perceive him as a threat. He allowed his suspicion to rule his heart. Now it mattered little what David did, it would never be right in King Saul's eyes. His failures would be highlighted. His successes would be scrutinized. If he did the wrong thing Saul was not surprised. If he did the right thing he had sinister motives. David was corrupt in Saul's eyes. But the real problem was Saul's eyes, not David's behavior. David was not doing anything wrong, Saul was simply seeing everything wrong.
1 Samuel 18:12 - Saul was afraid of David
Because David was suspect he was to be feared. He was a threat. He had to be addressed. Remember Saul actually attempted to kill David on several occasions. On the other hand David had at least a couple of opportunities to kill Saul and refrained leaving vengeance to God. But from Saul's perspective the threat had to be eliminated. David was such a bad man that he needed to be dealt with in the harshest terms. There was only one problem. David was not a bad man it was Saul that had bad eyes. It became so bad that anyone that even appeared to aid or defend David became a fit object of Saul's scorn. Saul had a whole family of Priest killed for innocently helping David. But Saul was sure they were in on a conspiracy that did not exist. Saul even threw a spear at his own son! He spent the last years of his life hating David. He was preoccupied with bringing David down. And all of this toward a man that till the end saw him as the anointed of God and even lamented his death on the battlefield as a great loss to Israel.
1 Samuel 18:29 - Saul became David's enemy
Saul was not David's enemy because David had declared war on him, but because he had declared war on David. All David could do was run and keep space between him and he who had chosen to be his enemy. Saul had an enemy of his own making! He became enemies with one who wanted to do him the most honor. He had made an enemy out of one who would not even engage him on those terms. In Saul's eyes David was the enemy. In David's eyes Saul was the Lord's anointed.
Saul had a vision problem that was precipitated by his own sin and disobedience. Because he was not right he began to see others around him as not right. Even he who was most right, a man after God's own heart, was suspect, was to be feared, and was ultimately counted as an enemy.
The arrangement kept David on the run for fifteen years hiding from Saul, waiting on God and it kept Saul in a constant state of agitation knowing that David lived and that he was a better man than he (which he admitted at one point) and that he would likely one day assume the throne.