Albert Barnes Commentary 2 Samuel 12:31

"And he brought forth the people that were therein, and put them under saws, and under harrows of iron, and under axes of iron, and made them pass through the brickkiln: and thus did he unto all the cities of the children of Ammon. And David and all the people returned unto Jerusalem." — 2 Samuel 12:31 (ASV)

For the saw as an instrument of torture, compare to Hebrews 11:37.

Harrows of iron — Or rather, threshing-machines (Isaiah 28:27; Isaiah 41:15).

Axes — The word translated this way occurs only here and in 1 Chronicles 20:3. It evidently means some kind of cutting instrument.

Made them pass through the brick-kiln — This phrase is the one always used for the cruel practice of making children pass through the fire to Moloch. It is likely that David punished this idolatrous practice by inflicting something similar upon the worshippers of Moloch.

The cruelty of these executions reflects the brutal customs of the age. While provoked by the Ammonites' conduct (2 Samuel 10:1–4; 1 Samuel 11:1–2), this cruelty is utterly indefensible in the light of the Gospel.

If Rabbah was captured before David’s repentance, he may have been in an unusually harsh and severe state of mind. The painful memory of Uriah’s death would likely have embittered him and made him extremely irritable.