The book states Saul died for his unfaithfulness to the LORD, including not keeping the word of the LORD and seeking guidance from a medium (1 Chronicles 10:13). It adds that he did not seek the LORD, so the kingdom was turned over to David (1 Chronicles 10:14).
In 1 Chronicles 10:13–14, the narrator gives a closing verdict on Saul’s death. Saul’s end is explained as the outcome of his breach of loyalty to Yahweh: he did not keep Yahweh’s word and sought guidance from a medium instead of seeking Yahweh. These verses function as the narrator’s concluding explanation after the brief account of Saul’s defeat and death in 1 Chronicles 10. The chapter retells Saul’s downfall in a compressed way, then adds an explicit “why” that turns the story into a transition.
These verses give the narrator’s final explanation for Saul’s death and for the change in kingship. The text explicitly ties Saul’s end to “trespass” against Yahweh: he did not keep Yahweh’s word, he sought guidance through a medium, and he did not inquire of Yahweh.
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with 1 Chronicles 10:13, then read the surrounding section so the answer stays connected to the passage's flow.
13So Saul died for his trespass which he committed against Yahweh, because of the word of Yahweh, which he didn`t keep; and also because he asked counsel of one who had a familiar spirit, to inquire [thereby],