King Saul's sin was basically his way of life. Instead of relying on the LORD God for instructions - as he did during the various steps involved in his anointment (1 Samuel 11) - Saul sought out his own ways to do things. The impact of this was particularly severe in chapter 15. This was the second time that Saul disobeyed the LORD's instructions, having already been chastised for disobedience during the events of chapter 13. While Samuel rebuked Saul with the news that God had rejected the continuation of his kingdom, Saul could have repented and God would have changed his mind. However, Saul chose to continue his disobedience following the victory over the Amalekites in chapter 15, fully aware of the consequences of disobedience. So he rebelled unhesitatingly, which is why in chapter 15 verse 22 Samuel compared Saul's disobedience to "divination" (a sin which, ironically, Saul was vehemently against but to which he eventually resorts to in chapter 28).Saul never took the LORD's word seriously.
This consecutive act of disobedience despite the awareness of its consequences was what made the LORD reject King Saul fully (1 Samuel 15:23). Saul wasn't that way when he first met Samuel and was informed of his anointment as king (chapters 9 & 10). Saul was humble and quite aware of his unworthiness (chapter 9 verse 21). But as he settled into is role of kingship, things gradually changed.
Why did Jesus rebuke the Pharisees more harshly than he rebuked the tax collectors Levi and Zacchaeus as well as the prostitute brought for stoning? It wasn't because he condoned their sin, but because they showed willingness to repent and transform. The Pharisees, though, considered themselves perfect and justified themselves rather than acknowledging the sin of their self-righteous, stubborn ways. God hates all sin, but the sin He hates the most is that of refusing to acknowledge it and turn away from it. That leads to the dangerous point of no return which Saul, unfortunately, had gotten to. He was left by God to be taunted by the Devil (1 Samuel 16:14) which eventually caused his murderous jealousy to David (1 Samuel 18:9-11), his witchcraft (1 Samuel 28) and his tragic suicide (1 Samuel 31). He became a toy for the Devil, who mercilessly played with his emotions and made his life a living hell. Unfortunately, many namesake Christians and churchgoers today are in this situation.
At the start of his career, King Saul could be said to have been at the height of spirituality. There are evidences to support this claim in 1 Samuel 11, particularly verses 6 and 12, 13 (where Saul refuses to kill those who opposed him and thereby shows great spiritual maturity). He was a lot like David back then. But towards the end of his life, Saul became a lot like Judas Iscariot. Like Judas, King Saul was buried in an obscure location after being beheaded post-death by the Philistines (1 Samuel 31:8-13). Judas' body too was mutilated (possibly after his suicide on a tree) in a place devoted to the burial of foreigners (Matthew 27:7, Acts 1:18).
It is tragic that King Saul, a man of such a figure who was found to be "a head taller than any of the others" (1 Samuel 9:2) had to meet this fate. The Bible has always mentioned King Saul as a very brave man who valiantly fought the enemies of Israel, particularly the Philistines, Ammonites and Amalekites. He was appointed by God to "deliver my people from the hand of the Philistines", which he did (1 Samuel 9:16). But, while he carried out the tasks he was appointed for by God, he forgot God Himself.
This is where we should remember John's letter to Ephesus in the book of Revelation chapter 2 verses 1 to 7. The church at Ephesus carried out many acts for Christ, even sacrificially enduring hardships and getting rid of false teachers claiming to be "apostles" (Rev 2: 2,3). But the letter didn't end on a good note. That's because the Ephesians had lost their "first love" for Christ (Rev 2:4). That warmth was gone in their relationship with God. Everything that was being done was done as a duty rather than the result of affection towards God.
Our love for Christ needs to be kindled just like the relationship between a husband and his wife. A wife needs to want to be with the husband and feel his love and warmth, rather than mechanically carrying out the tasks of a wife. As the bride of Christ, we Christians need to have that same affection for our bridegroom. Good works come after that. Without the affection and warmth (first love), good works hold no value to God. King Saul did many great things for Israel, but his heart was far from God. Eventually, those good deeds gave way to rebellion and self-justification.