"And God sent an angel to Jerusalem to destroy it; but as he was about to destroy it, the Lord saw and was sorry over the calamity, and said to the destroying angel, “It is enough; now relax your hand.” And the angel of the Lord was standing by the threshing floor of Ornan the Jebusite," (1 Chron. 21:15).
This verse is admittedly a difficult one to analyze. David was moved to number the people of Israel, and in so doing he sinned against God. Because of what he did, the Lord gave David three options of how God's judgment for this sin would fall on Israel. David chose to fall into the hands of the Lord, so God sent a pestilence upon Israel and 70,000 people died (1 Chron. 21:14). Then, "...God sent an angel to Jerusalem to destroy it; but as he was about to destroy it, the Lord saw and was sorry over the calamity, and said to the destroying angel, 'It is enough; now relax your hand,'" (1 Chron. 21:15). There doesn't seem to be any reason to delay such an intercession, a lesson to be given to the people through a trial or calamity. It is a punishment issue, and for some reason the Lord tells the angel to stop before it destroys Israel. This text brings several questions to mind:
Did God not know His own heart and know that He would not want Jerusalem destroyed? If so, why would He send an angel that He would later recall when there doesn't seem to be any intercession involved, any prayers by anyone? Are we to conclude that God was relenting over the very destruction He had caused? Does God's emotional state change so that His actions change even though He has perfect knowledge of all things?
Open theism offers no better explanation for these questions than does classical theism. From the open theist position God knows all things present, including His own heart. Surely He would have known His own anger and would have been perfectly able to predict His own behavior, thereby relenting on the destruction of Jerusalem. Of course, it is most probable that the people of Israel were praying and the Lord heard those prayers and relented, but that isn't what the text says.
The only explanation I can offer is that these verses are portraying God as both judgmental and compassionate. Furthermore, God's compassion is demonstrated in His relenting to destroy Jerusalem, which God surely knew would be the future place of Christ's crucifixion, which may have had something to do with the Lord not destroying it.