Again we must remember that much of the scripture is an account of history and not everything is directed by or approved of by God. Here is a mixture of both right and wrong. Certainly according to The Law the worshipers of Baal were condemned to die but the way in which it was done is evil and filled with deceit. Jehu has his own ambitions and uses The Law as a tool to his own means. His actions in the temple speak loudly about his character, that he was able to perform the rituals of Baal worship while maintaining the the charade shows that he truly was not following The Lord. There were other kings, more in Judah than Israel, that stopped the worship of Baal but we're told they simply tore down the temples and burned the poles, nothing like the slaughter here. All of Jehu's actions here are based on lies, anytime you feel the need to lie about anything to anyone you're doing something wrong.