"But now your kingdom shall not continue. The Lord has sought out a man after his own heart, and the Lord has commanded him to be prince over his people, because you have not kept what the Lord commanded you.”
1 Samuel 13:14
With this simple declaration from Samuel, the Lord spoke an end to Saul's reign. It would still be years away from occurring, but eventually God would appoint David as the dynastic king over Israel. God would remove Saul and end any chance of Saul's descendents taking the throne. From David's line would come rulers over Israel and eventually a covenant made by God to David that would bring a "forever-king" Messiah to fulfill all of God's promises to the children of Israel and to the world.
Saul was impulsively rebellious against God, usurping priestly roles at a whim and disobeying God's direct explicit commands. He was the king who thought he could veto God's plans. Saul was wrong. God would have the last word. Saul's kingship would end, and a king who sought God would be installed on the throne instead.
Saul teaches us that none of us can over rule what God wants of us. Oh, we can do so with our limited abilities thinking we have done it, but the consequences are disastrous and just may well end us altogether! And in that moment, God's will and word procede to be done without us, as our wills find failure. How much better it is to just trust and obey God from the very beginning and all the way through!