The Bible records many accounts of Saul displaying jealousy and hatred toward David – who was his personal musician, armor-bearer, army captain, son-in-law and successor. Saul saw that David was “a man after God’s own heart”, and it made him all the more aware of his own failings and weaknesses.
Early on, Saul was described as “a man without equal” (1 Samuel 9:2). The whole book of 1 Samuel chronicles the changing character of Saul’s heart. Chapter 10:6-9 tells us that the Spirit of the Lord was upon Saul and he was given a new heart. As long as he served, feared and obeyed God, he and his kingdom would be blessed. If he rebelled against God’s commandments, God’s judgment would be against him and the nation (12:14). In the beginning, Saul was a popular, humble, strong leader and warrior. He honored God and followed the prophet Samuel’s counsel. He did not have evil intentions, but lacked proper respect and reverence for the Lord and His provisions. While he might have been desirous of serving God, he was also desirous of serving himself. Sadly, he began to assert his own will and disobeyed God’s instructions. Saul was always a free-will agent in full control of his choices, but we know that "A double-minded man is unstable in all his ways."(James 1:8)
When David was anointed, the Lord’s spirit filled David and departed from Saul. “And the Spirit of Jehovah turned aside from Saul, and a spirit of sadness from Jehovah terrified him” (16:14-Young’s Translation). This distressing spirit was permitted by the LORD. Thereafter, Saul viewed David with suspicion- rage so intense that he attempted murder. Viewing David as his enemy, he did everything he could to dishonor David (18:7-15; 20:34). Perhaps he feared David because he knew the LORD was with him.
Saul became increasingly isolated, arrogant, attention-seeking (22:8); and “spiritually blind”. Samuel was forced to advise him twice that his kingdom would not endure and that he had been replaced as King: “Thou hast done foolishly: thou hast not kept the commandment of the Lord” (13:8-14) and “…The Lord hath rent the kingdom of Israel from thee this day, and hath given it to a neighbor of thine, that is better than thou…” (15:10-31).
In the meantime, David was prospering because the Lord was with him. David went wherever Saul sent him, and behaved wisely (18:14). Even Saul’s righteous son, “… Jonathan made a covenant with David because he loved him as himself” (18:3). Because Jonathan was surrendered to God he could see the hand of the LORD upon David and was perfectly willing to set aside his own ambition to honor the LORD's choice. “… David behaved himself more wisely than all the servants of Saul. So his name was highly esteemed” (18:30). Twice, David spared Saul’s life – in the cave and in the wilderness!
Saul forgot that “…to obey is better than sacrifice” (15:17-22). He did not properly receive the lessons the Lord provided and neglected his opportunity to be used in God’s service. King Saul hated David because David’s character exemplified everything Saul’s character did not.