Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
Nathan came to David as if to ask his judicial decision on the case about to be submitted to him (compare Sa2 14:2-11; Kg1 20:35-41). The circumstances of the story are exquisitely contrived to heighten the pity of David for the oppressed, and his indignation against the oppressor Sa1 25:13, Sa1 25:22.
Fourfold - The exact number prescribed by the Law (see the marginal references), and acted upon by Zaccheus. The Septuagint has "sevenfold," as in Pro 6:31.
And thy master's wives ... - According to Eastern custom, the royal harem was a part of the royal inheritance. The prophets spoke in such matters according to the received opinions of their day, and not always according to the abstract rule of right. (Compare Mat 19:4-9.)
See the marginal references. In both the points of David's crime the retribution was according to his sin. His adultery was punished by Absalom's outrage, his murder by the bloodshed of domestic fights, which cost the lives of at least three of his favorite sons, Amnon, Absalom, and Adonijah.
For a comment on David's words, read Ps. 51; Psa 32:1-11.
Thou shalt not die - Not spoken of the punishment of death as affixed to adultery by the Mosaic Law: the application of that law Lev 20:10; Deu 22:22; Joh 8:5 to an absolute Eastern monarch was out of the question. The death of the soul is meant (compare Eze 18:4, Eze 18:13, Eze 18:18).
The death of the infant child of one of the numerous harem of an Oriental monarch would in general be a matter of little moment to the father. The deep feeling shown by David on this occasion is both an indication of his affectionate and tender nature, and also a proof of the strength of his passion for Bath-sheba. He went into his most private chamber, his closet Mat 6:6, and "lay upon the earth" Sa2 13:31, rather "the ground," meaning the floor of his chamber as opposed to his couch.
Solomon - Or "peaceable," a name given to him at his circumcision. Compare Luk 1:59. The giving of the name Jedidiah, by the Lord through Nathan, signified God's favor to the child, as in the cases of Abraham, Sarah, and Israel. The name Jedidiah (which contains the same root as the name David, namely, "to love") indicated, prophetically, what God's Providence brought about actually, namely, the succession and glorious reign of Solomon over Israel.
The city of waters - The lower town of Rabbah (the modern Ammam), so called from a stream which rises within it and flows through it. The upper town with the citadel lay on a hill to the north of the stream, and was probably not tenable for any length of time after the supply of water was cut off.
Their king's crown - The word rendered their king (Malcham) is also the name of the national idol of the Ammonites (Jer 49:1, Jer 49:3 margin; Amo 1:15; Zep 1:5). Moreover, the weight of the crown, which is calculated to be equal to 100 or 125 pounds weight, is far too great for a man to wear. On the whole, it seems most probable that the idol Malcam is here meant.
For the saw as an implement of torture compare Heb 11:37.
Harrows of iron - Or rather thrashing-machines (Isa 28:27; Isa 41:15, etc.).
Axes - The word so rendered occurs only here and in Ch1 20:3. It evidently means some cutting instrument.
Made them pass through the brick-kiln - The phrase is that always used of the cruel process of making their children pass through the fire to Moloch, and it is likely that David punished this idolatrous practice by inflicting something similar upon the worshippers of Moloch. The cruelty of these executions belongs to the barbarous manners of the age, and was provoked by the conduct of the Ammonites Sa2 10:1-4; Sa1 11:1-2, but is utterly indefensible under the light of the Gospel. If Rabbah was taken before David's penitence, he may have been in an unusually harsh and severe frame of mind. The unpleasant recollection of Uriah's death would be likely to sour and irritate him to the utmost.