Matthew Poole and continuators on 2 Chronicles 20:35

35 And after this did Jehoshaphat king of Judah joyn himself with Ahaziah king of Israel, who did very wickedly :

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 threatned by a Prophet for the same thing, ch. 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 Jehoshaphats Temper, which could not resist the Solicitations of others, in such things as might seem indifferent. For he did not joyn with him in a War, as he did with Ahab, but in a peaceable way onely in a Matter of Trade and Commerce. And yet God sharply reproves and punisheth him for it, v. 37. to shew his great dislike and detestation of all friendly and familiar Conversation of his Servants and People with Idolaters and professed [illegible: 1+ letters]nemies of God and of Religion, as Ahaziah was, who is therefore thus stigmatized in the next words, who did very wickedly.

Or, who did industriously, and malitiously, and constantly work wickedness, as the Hebrew Phrase implyes, giving himself up to Idolatry and all Wickedness.