Charles Ellicott Commentary 2 Chronicles 15:13

"and that whosoever would not seek Jehovah, the God of Israel, should be put to death, whether small or great, whether man or woman." — 2 Chronicles 15:13 (ASV)

That. — And.

Whosoever would not seek the Lord God ... —Part of the solemn oath of the king and people; a sanction prescribed by the law of Deuteronomy 13:6 and following, Deuteronomy 17:2–6.