I hesitate to recommend Kuroshitsuji because in many ways it's a pretty problematic manga, but if you enjoy messy, toxic relationships, it definitely has those in spades.
When Ciel first arrived Hakkai tried to go "I don't think it's healthy for either of you to spend so much time together, I'll be restricting how much time you get with each othe--" Sebastian: "If you do that I will kill you and all your loved ones repeatedly : )"