1 Samuel 18:17

And Saul said to David, Behold my elder daughter Merab, &c. He at last bethinks himself of the promise he had publicly made unto him that should kill Goliath; the performance of which David did not demand, but in modesty left it to Saul's own conscience; who now judges it would be a proper bait to be laid for his destruction. “David had been very successful; but it did not follow that he must always be so; he had prudence, prowess, and conduct; but all these are often disappointed and defeated in their best-laid schemes. What means, then, so likely to destroy him as flattering him in his good fortune, and inflaming his vanity to yet higher and bolder attempts? What human heart is proof against flattery well conducted? and what so likely to point it right as the prospect of the king's alliance? Merab, therefore, the king's eldest daughter, is promised to him in marriage, on condition of his exerting all his fortitude in the defence of his master and his country, against the enemies of God and them.” Delaney. Only be thou valiant for me Thus, at the same time that he proposed to give David his daughter, he intimated that he should first perform some other military exploits, and, to give the better colour to this request, he calls it fighting the Lord's battles. Let not my hand be upon him Now he seems to have some sense of honour, and to lay aside those base thoughts of murdering him himself. But the hand of the Philistines By whose hand God's just judgment so ordered things that Saul himself fell!