Saul was afraid of David, because the LORD was with David but had departed from Saul.
References for 1 Samuel 18:12
So he sent David away from him and gave him command over a thousand men, and David led the troops in their campaigns.
References for 1 Samuel 18:13
In everything he did he had great success, because the LORD was with him.
References for 1 Samuel 18:14
When Saul saw how successful he was, he was afraid of him.
But all Israel and Judah loved David, because he led them in their campaigns.
References for 1 Samuel 18:16
Saul said to David, “Here is my older daughter Merab. I will give her to you in marriage; only serve me bravely and fight the battles of the LORD.” For Saul said to himself, “I will not raise a hand against him. Let the Philistines do that!”
References for 1 Samuel 18:17
But David said to Saul, “Who am I, and what is my family or my clan in Israel, that I should become the king’s son-in-law?”
References for 1 Samuel 18:18
So when the time came for Merab, Saul’s daughter, to be given to David, she was given in marriage to Adriel of Meholah.
References for 1 Samuel 18:19
Now Saul’s daughter Michal was in love with David, and when they told Saul about it, he was pleased.
References for 1 Samuel 18:20
“I will give her to him,” he thought, “so that she may be a snare to him and so that the hand of the Philistines may be against him.” So Saul said to David, “Now you have a second opportunity to become my son-in-law.”
References for 1 Samuel 18:21
Then Saul ordered his attendants: “Speak to David privately and say, ‘Look, the king likes you, and his attendants all love you; now become his son-in-law.’ ”