In 12:1-18 we have Samuel's reminder. This reminder implies a comparison between Samuel and Saul. If we compare Saul with Samuel, we will be able to see what is truly of God's heart and for God's heart.
First, Samuel reminded Israel of his integrity—that he was pure and just in dealing with the people (vv. 1-5). He reminded them that he had not taken anything from them, defrauded them, oppressed them, or taken any bribe from them. He said, "Jehovah is Witness against you, and His anointed is witness this day, that you have not found anything in my hand" (v. 5a). Samuel's word implied that Saul would pervert justice and take away many things from the people. Samuel had this kind of foresight concerning Saul.
In his wisdom, Samuel did not first speak about God to the people of Israel. Instead, Samuel reminded the people of his integrity and then went on to speak to them of Jehovah's righteous acts for Israel. He reminded them that it was Jehovah who appointed Moses and Aaron and who brought their fathers up out of the land of Egypt (v. 6). Then he reminded them that Jehovah delivered them from the hand of their enemies all around (v. 11).
Finally, Samuel reminded Israel of their wickedness in asking for a king, though Jehovah their God was their King (vv. 12-18). Jehovah was their King, yet they wanted another king (v. 12). Here Samuel was rebuking Israel for leaving her Husband. Because they wanted a king as a replacement for God, they would not have the proper enjoyment of the good land.
I believe that, to some extent at least, Samuel's intent here was to provoke the children of Israel. After Saul's conquest of the Ammonites, the people were happy, but they had been deceived by Saul, and Samuel was not happy about this situation. Samuel seemed to be saying to Israel, "You should have remained under the kingship of God, taking Him as your Husband and your Head. Then you would have enjoyed a good life, for He would have had His kingdom among you. But you have done wrong in taking a man as your head instead of God. You have actually divorced God and married Saul. You regard him as a handsome man, a man of choice, a man who is kind and humble. But this is only his outward appearance. Within him there is something dark. He will cause you to suffer and will take away many things from you."