In the original in universe novel plot line he was the main cause of several of the characters dying, being ousted or being severely injured by either using them as human shields for the original female lead or playing the boys who liked her off against each other. He hasn't done that yet in this timeline but with the way he was still setting things up so he could use the black haired duke it could have easily gone the same way if mc hadn't made Eugene and the Duke friends/brought folks to her side before ariel met any of them.
The crown prince's yandere, violent possessive behaviour is most likely something the main character (maybe accidentally considering how much of a goof she is) counteracts by helping him form a healthier relationship with the original female lead ariel. Plus the boys and supporting cast girls aren't being manipulated by him bc they're already in mc's group.
Also the possessiveness he feels is not limited to ariel so, it extends to everyone he feels is his or in his camp, like any aides, servants, etc. So... Eventually he likes mc enough that he considers her as "his" or "his person". Its a twisted reason to treat her better but hey at least he doesn't kill her I suppose.
So yeah, check out novel updates spoiler thread for the series if you want more spoilers for the series