Saul envied David and that envy moved Saul to attempt to kill David. 
Therefore, David eventually fled the land of Israel and joined Israel’s enemy, the Philistines. Sadder still was when the Philistines decided to fight Israel, the king of the Philistines wanted David with him, and David wanted to be with him,

“Now it happened in those days that the Philistines gathered their armies together for war, to fight with Israel. And Achish said to David, ‘You assuredly know that you will go out with me to battle, you and your men.’ So David said to Achish, ‘Surely you know what your servant can do.’ And Achish said to David, ‘Therefore I will make you one of my chief guardians forever’” (1 Samuel 28:1-2). 

Saul never knew the prize he had in his kingdom, and he likewise never knew the prize he drove away from Israel, all because of his envy and jealousy of David. 

Are you envious or jealous of someone?