John Gill Commentary 1 Chronicles 5:25

"And they trespassed against the God of their fathers, and played the harlot after the gods of the peoples of the land, whom God destroyed before them." — 1 Chronicles 5:25 (ASV)

And they transgressed against the God their fathers
Against his law, will, word, and ordinances, not only the half tribe of Manasseh, but the Reubenites and Gadites also:

and went a whoring after the gods of the people of the land, whom God
destroyed before them ;
that is, committed idolatry, which is spiritual fornication or whoredom; worshipped the idols either of the Amorites, who were destroyed by the Lord to make way for their first settlement; or of the Ishmaelites, whom they conquered, and whose land they dwelt in to the captivity.