Perhaps Jin dislikes Jian being overly concerned about Jitae. This is just my speculation, but the reason Jian is still in the Baek family might be because of Jitae. It could stem from guilt or perhaps some unresolved issues. Either way, it seems that Jian does things harmful to himself, such as associating with Hajoon (mentioned in chapter 19), because of Jitae. Jin clearly has an obsessive level of concern for Jian, so I guess Jin thinks the only way for Jian to completely sever ties with the Baek family is to make Jian deeply hurt by Jitae. At that point, he might abandon whatever plans he has and leave everything behind to be with Jin. I’m not excusing Jin’s actions, as they are still very toxic and controlling, but I assume that’s Jin’s intention.
I don't understand why Jin said that Jitae has to be the one to destroy Jian... Why him?