1 Chronicles 10:14

And enquired not of the LORD: therefore he slew him, and turned the kingdom unto David the son of Jesse.

Saul's death came in part because he did not inquire of the Lord. Instead of turning to the Lord he turned to a familiar spirit for counsel. That resort to forbidden counsel, together with his failure to keep the word of the Lord, was disobedience that brought his rejection and destruction. For this cause the Lord slew him and turned the kingdom to David the son of Jesse.