1 Chronicles 10:13

So Saul died for his transgression
(See Gill on 1 Samuel 31:13) A violent and dishonourable death, which was suffered on account of the sins he was guilty of:

one was, which he committed against the Lord, even against the word of
the Lord, which he kept not;
both in not staying for Samuel the time appointed, and by sparing the Amalekites whom he was bid to destroy, ( 1 Samuel 13:13 1 Samuel 13:14 ) ( 1 Samuel 15:3 1 Samuel 15:9 )

and also for asking counsel of one that had a familiar spirit, to
inquire of it;
what he should do with respect to engaging in battle with the Philistines, ( 1 Samuel 28:8 1 Samuel 28:15 ) which to do was contrary to an express command of God, ( Leviticus 19:31 ) .