A message of divine anger directed at the nation of Moab highlights a shocking and unusual crime. Rather than addressing harm done directly to the Israelites, God holds Moab accountable for an act of extreme cruelty committed against another foreign nation.
The primary approach among commentators traces this event to a joint military campaign waged by Israel, Judah, and Edom against Moab. During this conflict, a highly significant captive fell into the hands of the Moabite king. While [רש״י] understands the victim to be the ruling king of Edom himself, many commentators explain that the captive was actually the king's firstborn son, the crown prince destined to inherit the throne [מצודת דוד, רד״ק, מלבי״ם, אברבנאל].
The captive suffered a horrifying fate. The commentators agree that his bones were burned completely until they were reduced to a fine, white ash resembling lime used for building [מצודת ציון, מצודת דוד, רד״ק, אברבנאל, ביאור שטיינזלץ]. Some note that this execution was carried out on the city wall as a burnt offering to the idols of Moab [אבן עזרא, אברבנאל]. The cruelty extended far beyond the killing itself, culminating in an act of extreme humiliation. Driven by revenge and a desire to disgrace his enemy, the king of Moab took the crushed bone ash and used it to plaster the walls of his own home [רש״י, רד״ק].
This raises a profound question: why would God punish Moab for attacking Edom, a nation that was itself often an enemy? One perspective suggests that the punishment stems from a fundamental moral standard. Abusing the dead and violating the dignity of royalty is a despicable and unforgivable wrong in its own right. Furthermore, as descendants of Isaac, the people of Edom are entitled to a basic level of respect [רש״י, אבן עזרא, ביאור שטיינזלץ].
However, a broader perspective reveals a historical chain of cause and effect that directly connects this event to the fate of the Israelites. According to this view, the brutal execution of the Edomite royal sparked a fierce and enduring rage within Edom directed squarely at Israel. The king of Edom blamed the Israelites for dragging him into a war that cost him his heir and for failing to rescue the captive from Moabite hands. This deep resentment shattered their existing peace treaty, transforming Edom into a cruel and bitter enemy for generations. Ultimately, Moab faces divine punishment because its barbaric act served as the catalyst for the countless wars and hardships that Edom and its allies later inflicted upon Israel [מצודת דוד, רד״ק, מלבי״ם, אברבנאל].