David would not let anyone spoil his heart toward God, not his brothers, not Saul, not his own people, and certainly not his cousin Ammon.
David honored God and did not destroy his relative, Ammon, or steal his land; but the unsaved do not understand the commands and promises of God. They are proud and treacherous and truce breakers.
David showed the mercy and kindness of God to his cousin Ammon, but enemies of God never understand the kindnesses and mercies of God. They spit on the man who tries to bring them a cup of cold water in Jesus' name.
David kept a tender heart toward God. He did not let his heart become bitter. The evil of his relatives, or Saul, or his own people, did not poison his life.
I want a heart like David! I expect the enemy will be the enemy, the relatives will be relatives, and the people of God will be sinners, but I want to keep my heart tender toward God. I want to be humble and not proud.
I want the patience of God! God's purposes for Ammon are worked out by God over thousands of years. Justice for Ammon may not have a final resolution until the coming of Christ. I can wait! If God can wait (and He is the offended party), I can wait too.
Ammon was the son of Lot. In kindness to Lot and perhaps to Abraham also, God gave Ammon a piece of land across the river from Israel. It was Ammon's land, and Israel was not to take it. The descendants of Ammon were relatives but not friends to Israel. If fact Ammon made himself an enemy of God, but relatives are relatives and promises are promises.
2 Samuel 10:2-3
And David said, "I will deal loyally with Hanun the son of Nahash, as his father dealt loyally with me."
So David sent by his servants to console him concerning his father. And David's servants came into the land of the Ammonites. But the princes of the Ammonites said to Hanun their lord, "Do you think, because David has sent comforters to you, that he is honoring your father? Has not David sent his servants to you to search the city and to spy it out and to overthrow it?"
And when you approach the territory of the people of Ammon, do not harass them or contend with them, for I will not give you any of the land of the people of Ammon as a possession, because I have given it to the sons of Lot for a possession.
"I have heard the taunting of Moab and the revilings of the sons of Ammon, With which they have taunted My people and become arrogant against their territory. Therefore, as I live," declares the LORD of hosts, The God of Israel, "Surely Moab will be like Sodom and the sons of Ammon like Gomorrah-- a place possessed by nettles and salt pits, and a perpetual desolation. The remnant of My people will plunder them and the remainder of My nation will inherit them. This they will have in return for their pride, because they have taunted and become arrogant against the people of the LORD of hosts. The LORD will be terrifying to them, for He will starve all the gods of the earth; and all the coastlands of the nations will bow down to Him, everyone from his own place.
For me to live is Christ.