1 Samuel 18:17 - The Popular Commentary by Paul E. Kretzmann

And Saul said to David, Behold my elder daughter Merab, her will I give thee to wife, in fufilling the promise which was well known to all the soldiers of the army, 1 Samuel 17:25; only be thou valiant for me, distinguished for courage, and fight the Lord's battles. This was not a condition, but an obligation laid upon David, Saul taking this opportunity to impress the younger man with his zeal for the people of God and with the necessity of thwarting the evil intentions of the heathen neighbors. "But behind this proper language of Canaan was hid Saul's cunning and wickedness towards David. " (Lange.) For Saul said, namely, within himself, this was the thought which urged him on, Let not mine hand be upon him, it would have been a dangerous matter for him to take the life of David outright, but let the hand of the Philistines be upon him; Saul hoped that David would at some time fall in battle, and that the desired end would thus be obtained.