20 Meanwhile, Saul's daughter Michal was in love with David. When Saul was told of this, he rubbed his hands in anticipation. "Ah, a second chance. I'll use Michal as bait to get David out where the Philistines will make short work of him." So again he said to David, "You're going to be my son-in-law." 21  22 Saul ordered his servants, "Get David off by himself and tell him, 'The king is very taken with you, and everyone at court loves you. Go ahead, become the king's son-in-law!'" 23 The king's servants told all this to David, but David held back. "What are you thinking of? I can't do that. I'm a nobody; I have nothing to offer." 24 When the servants reported David's response to Saul, he told them to tell David this: "The king isn't expecting any money from you; only this: Go kill a hundred Philistines and bring evidence of your vengeance on the king's behalf. Avenge the king on his enemies." (Saul expected David to be killed in action.) 25  26 On receiving this message, David was pleased. There was something he could do for the king that would qualify him to be his son-in-law! He lost no time but went right out, he and his men, killed the hundred Philistines, brought their evidence back in a sack, and counted it out before the king - mission completed! Saul gave Michal his daughter to David in marriage. 27 

Matthew Henry's Commentary on 1 Samuel 18:20-27

Commentary on 1 Samuel 18:12-30

(Read 1 Samuel 18:12-30)

For a long time David was kept in continual apprehension of falling by the hand of Saul, yet he persevered in meek and respectful behaviour towards his persecutor. How uncommon is such prudence and discretion, especially under insults and provocations! Let us inquire if we imitate this part of the exemplary character before us. Are we behaving wisely in all our ways? Is there no sinful omission, no rashness of spirit, nothing wrong in our conduct? Opposition and perverseness in others, will not excuse wrong tempers in us, but should increase our care, and attention to the duties of our station. Consider Him that endured contradiction of sinners against himself, lest ye be weary and faint in your minds, Hebrews 12:3. If David magnified the honour of being son-in-law to king Saul, how should we magnify the honour of being sons to the King of kings!