The difference between David and Saul was that Saul kept getting caught and he kept getting admonished and even exposed, and he would always be sorry and cry, but then he'd go right back and do it again! But David really repented and was humbled and changed, and he asked God to forgive him, and the Lord did, although he still had to receive some punishment before the people, to let the people know that God wouldn't let him get away without suffering some kind of punishment, because he deserved it.
David knew he was nothing, and that if it weren't for God, he'd be back as a shepherd on the hillside somewhere. It was only GOD Who made him what he was and he knew it. Saul also knew that at first: He was so shy that when Samuel came to anoint him he hid himself amongst the baggage, although he was head and shoulders above his brethren. (1Sam.10:21-23)
But after awhile he forgot that it was God and he became lifted up in pride and it went to his head and he thought that it was Saul who put himself where he was, he thought it was his own arm that had saved him and that it was his own brilliance that made him king. So God said to him, "When thou wast little in thine own eyes I did highly exalt thee!" (1Sam.15:17) But when he became exalted in his OWN eyes then God had to abase him. (Luk.14:11)--That's the difference between David and Saul!
And that really is what finally brought about Saul's downfall. He figured he was so smart that he could get along without the Lord, and he went ahead in his own strength and his own wisdom and his own rightness and leaned too much to his own understanding and disobeyed the Lord, till the Lord had to rebuke him, chastise him, and tell him he was going to lose the Kingdom as a result of his disobedience!
Saul was so rebellious and disobedient that he wound up putting on an air of self-righteousness and even sacrificing to God! He built an altar to sacrifice to the Lord before the battle to ask for God's blessing. He was going through all the motions, the mechanics of being righteous. But it was pure self-righteousness because he wasn't obeying the Lord or the voice of the prophet and he didn't wait for God and God's voice. So Samuel said, "Listen, it's better to OBEY than to sacrifice! It's better to hearken, to listen to the voice of God, than to burn the fat of rams in self-righteous sacrifice!" (1Sam.15:22)
Finally poor Saul got so far off the track that he wist not that the Spirit of God was even departed from him! He didn't even realize that things weren't as they had been, that he was no longer the man he used to be and that the blessings of God had departed! He kept right on trying to pretend that he was the king and right and doing the best he could and doing the right thing. His attitude was, "Well, why does God let this happen? Why does God allow this to be? Why does He let me be defeated, my family and my soldiers and my country?"--Blaming it on GOD instead of himself and his own rebellious wicked heart and evil disobedience and horrible sin of rebelliousness, which is as witchcraft and idolatry! (1Sam.15:23) (Continued in Part V The root of all sin -- Pride )