A great military victory quickly transforms into a profound spiritual and leadership failure when the Israelite army chooses to obey God's command only partially. Instead of destroying everything as instructed, the king and the people apply their own human judgment. They spare the enemy king and plunder the finest property, destroying only what they consider worthless. This act of selective obedience ultimately leads to a severe punishment for Saul, costing him his royal throne [רלב״ג].
The root of this failure lies in the act of sparing the enemy and the spoils. This compassion is not driven by simple emotional pity or a desire to save resources, but rather by an intellectual calculation. Saul and the people use their own logic to decide that destroying the valuable spoils is unjust. By relying on their own reasoning, they essentially try to outsmart God, revealing a weakness in their faith by assuming His prophet's instruction is not entirely right [מלבי״ם].
Their selective obedience begins with sparing the Amalekite king, likely because of his high royal status or his impressive bravery on the battlefield [ביאור שטיינזלץ]. After sparing him, they carefully sort through the captured property, keeping the absolute best of the livestock. There are various views on exactly which animals are kept. Some suggest these are animals with double the usual amount of meat and fat [רש״י, מצודת ציון], while others believe they are medium-quality animals, second only to the very best [מלבי״ם]. Another perspective is that they save year-old animals or second-borns [רד״ק]. Additionally, they keep the fat, well-fed sheep and rams [מצודת ציון, רד״ק, ביאור שטיינזלץ], or specifically sheep that are over a year old [רד״ק].
The choice to keep these prime animals does not necessarily stem from greed, but may actually be the result of a religious mistake. The people believe they should dedicate the captured wealth as a sacrifice to God. Ordinarily, there is a legal concern about using animals taken from foreigners for the altar, as they might have been used in a sinful manner. However, the Israelites know that owners carefully protect their finest, fattest animals from any damage or flaw to maintain their physical value. Therefore, they deliberately save the best sheep and cattle with the pure, yet entirely misguided, intention of offering them to God [אהבת יהונתן].
Unwilling to part with this high-quality wealth [מצודת ציון], the army only carries out the command of destruction on the working livestock and everyday property [רד״ק, מצודת ציון, ביאור שטיינזלץ] that is damaged, despised, and completely worthless [רש״י, מצודת ציון]. The end result is a calculated division of the spoils: the superior and average items are carefully preserved, while only the absolute worst is destroyed [מלבי״ם].