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.  Then came the word of the Lord unto Samuel, saying,  It repenteth me that I have set up Saul to be king: for he is turned back from following me, and hath not performed my commandments. And it grieved Samuel; and he cried unto the Lord all night.  And when Samuel rose early to meet Saul in the morning, it was told Samuel, saying, Saul came to Carmel, and, behold, he set him up a place, and is gone about, and passed on, and gone down to Gilgal.
You think you can go the wrong way and accomplish your goal!
2. He Had too much Pride
1 Samuel 15:15-17,20,22 (MSG)
"Only some Amalekite loot," said Saul. "The soldiers saved back a few of the choice cattle and sheep to offer up in sacrifice to God . But everything else we destroyed under the holy ban."  "Enough!" interrupted Samuel. "Let me tell you what God told me last night." Saul said, "Go ahead. Tell me."  And Samuel told him. "When you started out in this, you were nothing---and you knew it. Then God put you at the head of Israel---made you king over Israel. Then God sent you off to do a job for him, ordering you, 'Go and put those sinners, the Amalekites, under a holy ban. Go to war against them until you have totally wiped them out.' So why did you not obey God ? Why did you grab all this loot? Why, with God 's eyes on you all the time, did you brazenly carry out this evil?"  Saul defended himself. "What are you talking about? I did obey God . I did the job God set for me. I brought in King Agag and destroyed the Amalekites under the terms of the holy ban. So the soldiers saved back a few choice sheep and cattle from the holy ban for sacrifice to God at Gilgal---what's wrong with that?"  Then Samuel said, Do you think all God wants are sacrifices--- empty rituals just for show? He wants you to listen to him! Plain listening is the thing, not staging a lavish religious production. Not doing what God tells you is far worse than fooling around in the occult. Getting self-important around God is far worse than making deals with your dead ancestors. Because you said No to God 's command, he says No to your kingship.
1 Samuel 15:23 (KJV)
For rebellion is as the sin of witchcraft, and stubbornness is as iniquity and idolatry. Because thou hast rejected the word of the Lord , he hath also rejected thee from being king.
Samuel tells Saul in this context that "Rebellion is as the sin of witchcraft"
Rebellion is willingly and knowingly doing your own thing ...selfishness on exhibition!
Imposing your will and desires among others...that's what witchcraft is!
It's interesting to note that Saul in his decline ends up entreating The Witch of Endor
Because he had the spirit of witchcraft upon him...those with like spirits will always connect...
Lustful people will connect with lustful people
Carnal people will connect with carnal people
Negative people will connect with negative people
Sometimes it's good to look around you and see who your hanging out with because it defines who you are or who you will become!
4. He Refused to Take Responsibility For His Actions
You can never go and never be what God wants you to be until you take responsibility for your own actions.
Saul when pressed says "the people did it"
Lack of integrity always pushes responsibility!
Adam blamed Eve---Eve blamed the serpent
5. He Was More Concerned About What the People Thought Vs What God Thought
1 Samuel 15:24,26,30 (MSG)
Saul gave in and confessed, "I've sinned. I've trampled roughshod over God 's Word and your instructions. I cared more about pleasing the people. I let them tell me what to do. Oh, absolve me of my sin! Take my hand and lead me to the altar so I can worship God !"  But Samuel refused: "No, I can't come alongside you in this. You rejected God 's command. Now God has rejected you as king over Israel."  Saul tried again, "I have sinned. But don't abandon me! Support me with your presence before the leaders and the people. Come alongside me as I go back to worship God ."