Albert Barnes 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)

For his transgression - Compare 1 Chronicles 9:1. The “transgression” intended is probably the disobedience regarding Amalek, recorded in 1 Samuel 15:1–9 (compare 1 Samuel 28:17–18).