He consulted a medium and did not seek guidance from the Lord. "So Saul died for his unfaithfulness; he was unfaithful to the Lord in that he did not keep the command of the Lord" (1 Chronicles 10:13), and the verse goes on to say he consulted a medium. 1 Samuel 31 tells the death with no verdict at all, and 1 Samuel 28:6 says Saul did ask the Lord and got no answer, "not by dreams or by Urim or by prophets."
The verdict is typical of Chronicles, which spells out why a king was rewarded or punished. Its two key words, unfaithful and seek, are favorites of the book. Of all the Saul stories in 1 Samuel, Chronicles tells only his death, and it assumes the reader already knows the rest.
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