John Gill Commentary 2 Kings 8:18

"And he walked in the way of the kings of Israel, as did the house of Ahab: for he had the daughter of Ahab to wife; and he did that which was evil in the sight of Jehovah." — 2 Kings 8:18 (ASV)

And he walked in the way of the king's of Israel, as did the
house of Ahab
Imitated them in idolatry:

for the daughter of Ahab was his wife ;
whose name was Athaliah, (2 Kings 8:26) , and by her he was drawn into idolatrous practices; of such bad consequence are marriages with idolaters; it is very much that so good a king as Jehoshaphat his father was should contract such an affinity; he suffered for it in more instances than one:

and he did evil in the sight of the Lord ;
was guilty of idolatry, than which nothing was more displeasing to the Lord; for he made high places, and compelled his subjects to commit idolatry, (2 Chronicles 21:11) .