After experiencing great salvation and divine intervention, a righteous leader can still tragically fall into the trap of pride. This misstep not only affects the individual but can bring divine anger upon an entire nation. Despite receiving immense kindness and reward from God, such as a miraculous recovery from a severe illness, Hezekiah failed to repay God with corresponding goodness. The root of this failure was a sense of arrogance that began to grow in his heart.
The commentators offer complementary explanations for how this pride took shape. The primary approach among commentators points to the arrival of messengers from the king of Babylon. Instead of remaining humble, Hezekiah chose to show all his treasures to them, driven by a desire to boast about his wealth [רש״י, מצודת דוד]. Taking a different angle, another perspective suggests that the very act of amassing such massive treasures of silver and gold was the core issue. This behavior directly goes against the Torah's rule forbidding a king from accumulating excessive wealth, a law specifically designed to prevent a leader from becoming haughty [מלבי״ם].
An additional layer of this pride was Hezekiah's internal attitude toward his successes. He either attributed the military victory over Assyria to his own strength, or he interpreted God's miracles as a mark of his own personal glory. He failed to recognize that the true purpose of these divine wonders was to elevate the standing of Israel and instill fear in their enemies [מלבי״ם, ביאור שטיינזלץ].
As a direct result of this prideful behavior, divine anger was awakened against him [ביאור שטיינזלץ]. This anger did not stop with the king alone but extended to the entire nation of Judah and the city of Jerusalem. The historical account in the Book of Chronicles keeps the description of this anger and the resulting punishment remarkably brief. This concise approach is taken because the full details of the event and its broader consequences were already recorded at length in the Books of Kings and Isaiah [רש״י].