The first monarchy in Israel reaches its tragic breaking point precisely in the aftermath of a military victory. Rather than fully carrying out the divine command, the king and the people turn their attention to the spoils of war. At this critical moment, God delivers a message that seals the fate of the king's rule, expressing His regret over making Saul king.
From a historical and spiritual perspective, the monarchy was always intended for David. Saul was appointed merely as a temporary answer to the people's early demand for a leader. Because his rule was not meant to be permanent, this single failure was enough for the kingdom to be taken from him and given to its original destiny. This stands in contrast to David, who was able to keep his throne even after he sinned [אהבת יהונתן].
God points to two separate offenses that lead to this final break. The first is that Saul turned back and did not follow His direction, which points to his earlier failure at Gilgal. The second is his failure to carry out God's word in the current conflict against Amalek [אברבנאל].
Upon hearing this harsh news, Samuel spends the entire night crying out in prayer. His intense pain and anger come from several places. He had a deep love for Saul, a mighty and handsome warrior whom he had personally anointed, feeling the kind of attachment an artist feels toward his creation. Moreover, while Saul's first failure at Gilgal could have been forgiven through prayer, repeating the mistake eliminated any chance of fixing it. Finally, God only revealed the punishment without explaining the exact details of the sin, leaving Samuel to cry out in the dark to understand what action had sealed the king's fate [אברבנאל].
Early the next morning, Samuel goes to confront Saul. He finds him at Carmel, setting up a place either to divide the loot or to build an altar. There, Samuel delivers the harshest possible rebuke. He warns the king that obeying God is far more important and desired than bringing sacrifices, and that refusing to listen to His voice is equal to the sin of idol worship [צאינה וראינה, אברבנאל].