Saul said to David, Behold, my elder daughter Merab, her will I give you as wife: only be valiant for me, and fight Yahweh's battles. For Saul said, Don't let my hand be on him, but let the hand of the Philistines be on him.
"Let the hand of the Philistines be upon him"  (1 Samuel 18:17).  So, it is clear enough.  Saul's promotion of David in the army and the assignment of military campaigns to him was done, so Saul hoped, that it might result in David's being killed in battle.