I Samuel 18:21-22, 29

So Saul said, “I will give her to him, that she may be a snare to him, and that the hand of the Philistines may be against him.” Therefore Saul said to David a second time, “You shall be my son-in-law today.” And Saul commanded his servants, “Communicate with David secretly, and say, ‘Look, the king has delight in you, and all his servants love you. Now therefore, become the king’s son-in-law.’ ”

and Saul was still more afraid of David. So Saul became David’s enemy continually.