The difference between Saul and David can be seen in 1 Samuel 23. Enemies attacked Keilah which was part of the Israelite ground. Yet Saul, who was then king, was more concerned to maintain his throne rather than dealing with the enemies who had conquered part of his land.
Let's take a closer look at Saul and David.
Saul:
Instead of dealing with the enemy, he diverted his armies to pursuit David. All he had in mind was to get rid of David to maintain his throne. V.7-8, 21.
There were no records of him seeking the Lord, but amazingly he blessed the Lord thinking that God had delivered David into his hand. V.21. A strange assumption indeed for a man whom Samuel told that David will be king. I call this Saul's syndrome.
David:
He was concerned about the real enemies of the Philistines. He sought the Lord as to whether he should go to Keilah to drive out the enemy. He acted only upon receiving instruction from the Lord. V.2,4 &10.
He left Keilah again when the Lord answered that the people of Keilah will deliver him to Saul.
God's protection was upon David even when Saul was very close to having to get him. The Philistine attacked and Saul has to turn other directions instead of pursuing David.
There was even a rock remembered by the people in remembrance of this event! V.28
We have to be careful of Saul's Syndrome. It is a deadly mistake anyone can make. Saul's syndrome is to assume God's hand is upon something when it seemed convenient. It does not mean taking the route of inconvenience being the Lord's way either.
The key here is to seek the Lord and only where He shows do we follow.