Charles Ellicott Commentary 2 Samuel 21:8

"But the king took the two sons of Rizpah the daughter of Aiah, whom she bare unto Saul, Armoni and Mephibosheth; and the five sons of Michal the daughter of Saul, whom she bare to Adriel the son of Barzillai the Meholathite:" — 2 Samuel 21:8 (ASV)

Took the two sons of Rizpah.—The suggestion that David took advantage of this opportunity to strengthen himself further against the house of Saul is utterly set aside by two considerations:

1. David could not lawfully refuse the demand of the Gibeonites, since the Law absolutely required that blood-guiltiness should be expiated by the blood of the offender (Numbers 35:33), which, in this case, became that of his representatives.
2. David’s choice of victims was directly opposed to such a supposition.

He spared, for Jonathan’s sake, the only descendants of Saul in the male line, who only could have advanced any claim to the throne, and took (1) the two sons of Rizpah, a concubine of Saul, with whom Abner had committed adultery (2 Samuel 3:7), and (2) five sons of Saul’s eldest daughter Merab, who had been promised in marriage to David himself, and then given to another (1 Samuel 18:17–19). The text has Michal instead of Merab; but this must be an error of the scribe, since it was Merab, not Michal, who was married to Adriel the Meholathite (1 Samuel 18:19), and Michal was childless (2 Samuel 6:23). The English phrase “brought up for” is taken from the Chaldee; the Hebrew, as noted in the margin, is bare to.