1 Samuel 18:12

And Saul was afraid of David, because the LORD was with him, and was departed from Saul.

Saul was afraid of David because the Lord was with David and had departed from Saul, so that David stood divinely favored while Saul was left abandoned to his own guilty course. His jealousy and fear kept growing as David won the love and confidence of the people, and Saul feared that David would take the kingdom from him. Although Saul wanted to kill David, he was afraid to move against him openly because the people highly esteemed David.