Chapter 15 is the turning point in the life and reign of Saul, as Samuel declares, "Because you have rejected the word of the Lord, he has also rejected you from being king" (v 23). The chapter ends with tears and regret, "Samuel grieved over Saul. And the Lord regretted that he had made Saul king over Israel" (v 35).
Saul's last chance hinged on his obedience to a clear command, a command Saul heard and understood and disregarded. Saul's own words convict him, "I have obeyed the voice of the Lord. I have gone on the mission on which the Lord sent me. I have brought Agag the king of Amalek, and I have devoted the Amalekites to destruction. But the people took of the spoil, sheep and oxen, the best of the things devoted to destruction, to sacrifice to the Lord your God in Gilgal" (vv 20-21). Saul is passing the blame to the people, but Saul is king, and it was he who kept "the best of the sheep and of the oxen and of the fattened calves and the lambs, and all that was good" (v 9).
When God devoted the Amalekites to destruction, God meant everything. The Amalekites were a particularly wicked people, avowed enemies of Israel (see Deuteronomy 25:17-19). God's covenant with Abraham to make him a great nation included the promise, "I will bless those who bless you, and him who dishonors you I will curse" (Genesis 12:3). The command to Saul was God keeping his promise to Israel. It was a just God dealing with a wicked and evil enemy.
Saul turned God's justice into plunder for profit. Saul would destroy what was not worth keeping and keep the best. He knew exactly what God commanded, but he thought better and did less. The nation of Israel is called to "love the Lord your God with all your heart and with all your soul and with all your might" (Deuteronomy 6:5). Saul bargained for a percentage, "I'll give you 80%, God." The 20% Saul kept was his way of showing who was really king. It exposed his heart, and Saul now spoke of the Lord as "your God" (1Samuel 15:21), not his.
Partial obedience is disobedience, and Saul served at God's pleasure. God was no longer pleased, "Has the Lord as great delight in burnt offerings and sacrifices, as in obeying the voice of the Lord? Behold, to obey is better than sacrifice" (1Samuel 15:22).
Almighty God, You are awesome. I use that word too often for other things and diminish the word. It belongs to You. I am in awe of Your creation, Your mighty power, Your majesty, Your holiness, and in the midst of all that, Your patience with me.
You are powerful and loving, strength under control, gentle and mighty. You are amazing, and it is You and You alone I worship and follow, fully and always.