1 Samuel 15:9
But Saul and the people spared Agag, and the best of the sheep, and of the oxen, and of the fatlings, and the lambs, and all that was good, and would not utterly destroy them: but every thing that was vile and refuse, that they destroyed utterly.
God's command to destroy Amalek belonged to a judgment long pronounced through Moses because the Amalekites had first attacked Israel in the wilderness, defied God, and practiced idolatry. That sentence had been delayed for four hundred years, but the Amalekites had not turned from their sins, and the time had come for it to be carried out.
What followed was selective obedience and incomplete execution, for Saul and the people spared Agag and the best of the sheep and oxen and fatlings and lambs and all that was good, while they utterly destroyed only what was vile and worthless and refuse. They destroyed what was inferior and valueless yet kept the captive king and the finest animals and everything valuable, obeying only in part. Saul spared Agag to enhance the honor of his triumph, to bring himself renown, and to make other nations fear him. The keeping of the finest livestock was excused as an offering to God, but the intent was to use those animals for sacrifice instead of their own and thus save their own livestock. Saul later blamed the people for this and claimed to have carried out the command, using the sacrifice plea to excuse disobedience.
Such partial obedience does not count as obedience. God had cursed Amalek and ordered its destruction, so they were not to keep the spoil for themselves, to gain glory, or to offer it as a sacrifice, but to carry out God's word concerning Amalek. By failing they prevented the nations from seeing Amalek destroyed by the people it had despised. In sparing what was commanded to be destroyed, Saul substituted his own judgment for God's explicit command, showing that he did not treat God's requirements as supreme or trust God under difficulty.
By that act Saul led his people to feel they also might follow their own judgment instead of obeying God, failing to see that their prosperity as individuals and as a nation depended on strict adherence to the command of Him who sees the end from the beginning. Loyalty to God is proved by unquestioning obedience, and in deciding any course the question is not whether harm can be seen to result from it but whether it is contrary to the will of God. Saul's failure to do this work of justice in carrying out God's judgment on Amalek kindled God's wrath. Saul then refused to acknowledge his wrongdoing, and his pattern is repeated when a person denies fault until the evidence is undeniable, yet still makes no confession and instead considers himself mistreated.