What Chesed did Nachash do with David?
'Rashi', Rashi (Shmuel II, 10:2, citing Tanchuma Vayera 25): When David fled from Sha'ul, his parents and brothers came to him, and he left them with Melech Mo'av (Shmuel I, 22:4). Melech Mo'av killed all of them, except for one brother, who fled to Nachash in Eretz Amon, who kept him alive.
Why did David send only after Nachash died?
Malbim (Shmuel II, 10:2): Had he sent in his lifetime, he would have needed to make a Bris with the entire nation, which is forbidden.
It says "Lo Sidrosh Shelomam v'Tovasam" (Devarim 23:7). Was it proper to send to Chanun?
Radak (Shmuel II, 10:2, citing Tanchuma Pinchas 3): No. He was punished; he sent to them with mercy, and came to disgrace (Chanun shaved half the beards of the Sheluchim and tore their garments - verse 4), and David needed to fight against four nations (in this Perek). Hashem rebuked David - "Al Tehi Tzadik Harbeh" (Koheles 7:16)! Due to Chanun's act, David disgraced Mo'av more than his other enemies (Shmuel II, 8:2).
Malbim citing Hagahos Maimoniyos (Hilchos Melachim 6:6, citing Re'em): Yes. If an Amoni or Mo'avi did Chesed for you, you may repay Shalom. Malbim - this applies also to consoling him over the loss of his father. This honors his father; it is like returning Chesed. David was punished (refer to 10:2:4:1) because it looked like his Sheluchim went "Eretz Bnei Amon", to seek the Shalom of the entire nation.
