But if he's being manipulated, is it really his choice?
Sooyoung knows that Taesung isn't being straight with him, that he's lying about certain things so he isn't as naive as some people assume. I think Sooyoung's choice will ultimately be will he tolerate the way that Taesung is and the path that their relationship is on. And I don't think Sooyoung will accept Taesung as he is now. The moment he finds out that Taesung incidently or not killed his uncle that's their crash point. Sooyoung is a lot of things but he still has his pride and he wouldn't tolerate Taesung trying to sweep that away. You know?
If anyone thinks that anyone in this story is mentally stable they've got another thing coming.
In a realistic world, Sooyoung wouldn't be involved with either of them. But this is the universe that the author has created and he's had his sense knocked out of him more than once.
Sooyoung likes Taesung. It's a terrible choice but okay. Minhyuk needed to back off but he is trash of a different sort. If Sooyoung decides to forgive him (IDK if this could even happen) then we have to accept that too because that's the story set out before us.
Raging at the immorality of the story is extra stress you don't need.
Sooyoung may end up with Taesung but he is aware enough to know what that means if he makes that choice. Personally, I don't see it. Sooyoung is a strong person and Taesung will break sooner than he does.
TL; DR Support Sooyoung's choices, don't think about it too much.