John Gill Commentary 1 Chronicles 10:13

"So Saul died for his trespass which he committed against Jehovah, because of the word of Jehovah, which he kept not; and also for that he asked counsel of one that had a familiar spirit, to inquire [thereby]," — 1 Chronicles 10:13 (ASV)

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) .