2 Chronicles 20:35 Commentary - English Annotations on the Holy Bible by Matthew Poole

This is mentioned as an aggravation of his sin, after so great a favour and obligation laid upon him by God, and after he had been so sharply reproved and threatened by a prophet for the same thing, 2Ch 19:2; yet he relapsed into the same sin; which proceeded partly from that near relation which was contracted between the two families, and partly from the sweetness and easiness of Jehoshaphat’s temper, which could not resist the solicitations of others in such things as might seem indifferent. For he did not join with him in a war, as he did with Ahab, but in a peaceable way only, in a matter of trade and commerce. And yet God sharply reproves and punisheth him for it, 2Ch 20:37, to show his great dislike and detestation of all friendly and familiar conversation of his servants and people with idolaters and professed enemies of God and of religion, as Ahaziah was, who is therefore thus stigmatized in the next words,

who did very wickedly. Who did very wickedly, or, who did industriously, and maliciously, and constantly work wickedness, as the Hebrew phrase implies, giving himself up to idolatry and all wickedness.