Charles Ellicott Commentary 1 Samuel 24:21

"Swear now therefore unto me by Jehovah, that thou wilt not cut off my seed after me, and that thou wilt not destroy my name out of my father`s house." — 1 Samuel 24:21 (ASV)

Swear now therefore unto me. —Saul was so strongly convinced at this moment that David would before long occupy the throne of Israel that he entreated him, when that day would come, not to destroy all his (Saul’s) children. This barbarous custom has always been an all too common practice in the jealous East.

It seems to have been equally dreaded by Jonathan, who made—it will be remembered—this condition of mercy to be shown by David in his day of power to his (Jonathan’s) children a part of the solemn covenant concluded between them (see 1 Samuel 20:15). In the frequent dynastic changes that took place in the kingdom of Israel, we have instances of such wholesale massacres of the royal family of the fallen house (see 1 Kings 15:29, where Baasha slew King Nadab, the son of Jeroboam, and took his throne).

Then Baasha, we read, smote all the house of Jeroboam; he left not to Jeroboam any that breathed; and in 1 Kings 16:11, where Zimri murdered his master, King Baasha, Zimri, as soon as he sat on his throne, slew all the house of Baasha: he left him not one, neither of his kinsfolks, nor of his friends. A similar massacre is described, only with more ghastly details, in 2 Kings 10, where Jehu slew all that remained to Ahab in Samaria. There the story is a particularly Oriental scene of history, with the seventy baskets containing the seventy heads of princes presented as an acceptable offering to the new, stern king of Israel—Jehu.

It was, therefore, no vain dread of what might happen in the future that made King Saul ask this of David. Doubtless, the fear of some such awful catastrophe happening to his own loved children and friends was no small part of the punishment of Saul.