Theres a difference between well developed drama and throwing in a random malicious element in the third chapter. The curse itself is already a problem for their relationship, so it's perfectly reasonable for people to be bothered by another unpleasant element being introduced so soon. A drama tag does not automatically mean that everyone who reads it needs to be okay with third wheels popping out of the woodwork and fucking up a developing story. Im sure the MC will pass the "test" but another guy (?) -who's clearly selfish, manipulative, and a bit fucked up- having that much influence over the FMC so early on is concerning for everyone who wants the main couple to have a happy ending.
It's also doubly unpleasant because that cliche character type is always ridiculously entitled, with some sort of sense of ownership of romance interest that theyre friends with - It's a gross character type, flat out.
Remember most of these stories --where 5 guys feel obligated dverytime to go "Hurr Durr there's a Drama tag" the moment anyone shows any amount of reaction to a development-- are meant to be dramatic romances, not romantic dramas, there is a difference.
What's immature is not understanding that people who are empathizing with the MC are against s clear secondary antagonist who does not have the MCs best interests at heart (for example if the new douche character really convinced the girl to split with the MC then the MC would likely die since hes basically placed the rest of his life in her hands) and therefore expressing ire with the development displays empathy, and is not something to be criticized.
(And honestly what do expect people to do? Cheer that a creepy manipulative weirdo showed up?) >> Thats a rhetorical question. Meaning the answer is apparent so if you feel the need to "hurrdurrdramatag" at me in response, just dont. I'm not gonna bother checking.