26 Now Joab fought against Rabbah of the Ammonites and took the royal city. 27 And Joab sent messengers to David and said, "I have fought against Rabbah; moreover, I have taken the city of waters. 28 Now then gather the rest of the people together and encamp against the city and take it, lest I take the city and it be called by my name." 29 So David gathered all the people together and went to Rabbah and fought against it and took it. 30 And he took the crown of their king from his head. The weight of it was a talent[1] of gold, and in it was a precious stone, and it was placed on David's head. And he brought out the spoil of the city, a very great amount.
Matthew Henry's Commentary on 2 Samuel 12:26-30
Commentary on 2 Samuel 12:26-31
(Read 2 Samuel 12:26-31)
To be thus severe in putting the children of Ammon to slavery was a sign that David's heart was not yet made soft by repentance, at the time when this took place. We shall be most compassionate, kind, and forgiving to others, when we most feel our need of the Lord's forgiving love, and taste the sweetness of it in our own souls.