Q. I always felt sorry for Saul. God chose him to lead His people, and he did a good job at it. Saul only made one mistake and God sent David to replace him. I think David did much worse, yet God said, “He’s a man after my own heart.”

These are excellent questions. In this post I’ll look at why God rejected Saul as king. In my next post I’ll consider how God could call David a “man after my own heart.”
Kingship in Israel was supposed to be different from kingship in the surrounding nations. Israel’s king was not to be considered divine. In the law of Moses, God carefully distinguished the priesthood from the kingship and gave future kings careful instructions that put them under the law.
So it was vital that Israelite kings not usurp any priestly or divine prerogatives. The precedent that Saul set as Israel’s first king would influence all of his successors (like George Washington declining a third term). So he was held to a strict standard.
At one point during Saul’s reign, he was campaigning against the Philistines and waiting for Samuel to come and offer sacrifices to seek God’s favor. When Samuel didn’t arrive as soon as he expected, Saul offered these sacrifices himself, assuming the prerogatives of a priest. When Samuel did arrive, he told Saul, “You have done a foolish thing,” using the Hebrew term for people who act without regard for God. Samuel warned that Saul’s kingdom would not endure, meaning that his family would not establish a dynasty. He’d be succeeded on the throne by someone from a different family.
Some time later, however, God gave Saul a new assignment in his capacity as king. Samuel introduces this assignment by saying, “I am the one the LORD sent to anoint you king over his people Israel.” So perhaps this was intended as a “second chance.”
God commanded Saul to destroy the Amalekites. (This is one of those episodes of total destruction in the Bible that are very difficult for us to understand; I’ve shared some thoughts about them here.) One thing we can recognize in such episodes is that the Israelites were never to take any plunder because weren’t in the war for themselves; they were considered agents of divine judgment.
But Saul and his army spared “the best of the sheep and cattle, the fat calves and lambs—everything that was good.” They only destroyed what they thought was undesirable and worthless. They spared King Agag because in this time captured kings were a prized trophy of war. By conducting this raid as if it were ordinary warfare that he was directing, Saul once again usurped a divine prerogative and misrepresented the character of divine judgment, which doesn’t privilege the powerful and the beautiful.
It seems that God gave Saul a second chance, but this only showed that he still hadn’t learned to respect the limits of his authority as king. And so, to prevent Israelite kingship from being established on the model of the divine kings or priest-kings of surrounding nations, God didn’t allow Saul to establish a dynasty.
Nevertheless, even after Samuel announced this judgment a second time, he granted Saul’s request, “Please honor me [as king] before the elders of my people and before Israel.” Saul reigned for 42 years and throughout that time he was acknowledged as the rightful king. David, even though promised the kingship himself, respected and protected him as the “LORD’s anointed.”
One of the last things we hear about Saul in the Bible is David’s tribute to him after he was killed in battle. Acknowledging how Saul had made Israel secure and prosperous by defeating its enemies, David laments,
Daughters of Israel, weep for Saul,
who clothed you in scarlet and finery,
who adorned your garments with ornaments of gold.
How the mighty have fallen in battle!
So even though Saul wasn’t able to establish an Israelite royal dynasty on the right principles, the Bible acknowledges the benefits Israel received from his long reign.