Never said all stories are predictable. There's plenty of very original stories out there, but THIS just isn't one of them. Which is why the direction was extremely predictable from the start if you've read more than a handful of BLs, especially smutty Yaois. It uses so many popular tropes, e.g. innocent uke x toxic badboy seme, dub-con fetish, starting a relationship with a sex deal and then developing feelings, misunderstandings driving them apart, love rivals coming in-between them, and many more.
Of course it was never gonna end with Cain as endgame, the guy is just a plot device to add more drama between the two actual main characters. While likeable, he's a severely underdeveloped character who doesn't have a plot or personality of his own. He wasn't even introduced until the end of season 1. An author that relies this heavily on tropes would never do that if they thought of Cain as a real main character and endgame, because the author WANTS the readers to root for the right guy.
If you're still a Yahwi hater, I suggest, you read the whole story again so that you'll know why Yahwi unintentionally did those things to Jooin. Although it wasn't really an excuse, it still contributed to his behavior. HE WAS MISUNDERSTOOD.
AND let's also not ignore the fact that it wasn't all Yahwi's fault why things went downhill. Both were on fault, so if ya'll can't forgive Yahwi for "abusing" Jooin, then you shouldn't be doing the same thing to Jooin.
Let me just remind ya'll that he treated both Cain and Yahwi like shit; by sleeping with Cain, leading him on and stuff knowing damn well he still got feelings for Yahwi.