Maybe for contrast between 2 opposite types of relationship.
Maybe the author thought by making a toxic secondary couple we would appreciate more of the relationship between the first couple.
Kinda feels like false advertising, though; imagine reading the synopsis and enjoying a healthy, cute couple for several chapters, and then we get a toxic couple out of nowhere. I almost quit the series because of that.
especially since one of the tags for this story is #wholesome it’s a jump scare to suddenly get a toxic ass couple in the mix
I'll take the "L" because I don't look at tags, but in this situation, yeah, that's messed up.
I'll never understand why the author advertised a wholesome BL and gave us a toxic secondary. If I wanted to stretch things, I could say it's supposed to be a parallel since the primary seme and secondary uke have homophobic pasts...but that's about it. Secondary Seme is insanely toxic and unsympathetic, so trying to make it a bittersweet story like it's love lost feels like gaslighting.