I wouldn’t really call Sangwoo a winner. He is experiencing a lot of pain through PTSD and worrying about Yoon Bum abandoning him, but he is just very good at deceiving and hiding his pain while Bum can’t do the same. It wasn’t that Sangwoo refused ton back down, but he broke and killed his parents while Yoon Bum’s breaking was a bit different, you can even almost say Bum was stronger since he didn’t resort to something so primal to cope with his trauma.
Sangwoo didn’t build a good life for himself, he had “a good life” already. He had a house, he had food, he had a car, his family had money and he just had very shitty parents to add to that.
Bum on the other hand was broke to the point where his uncle had to beg a woman so he doesn’t have to pay a bit more than two thousand dollars to her.
They were both beat, yes, but Yoon Bum lived through it while Sangwoo turned things around for himself and became unhealthily “proper”.
Between the two of them, it’s actually Sangwoo who is broken to the point where he doesn’t function as a human at all anymore while Bum has moments of clarity and has wavering morality. If anything, Bum is the winner for being able to stay as a human while suffering from all of this, even as it was painful.
Sangwoo on the other hand just gave everything up and his inner child shrivelled inside him and hid somewhere. Something inside him broke irreversibly. Sangwoo lost the ability to feel anything, whoever’s fault it was, and I really don’t think that counts as a win or a good life.
However it does seem like that, so you are sort of right.
Sangwoo looks like he is perfect. This is how Sangwoo tricks everyone into thinking he is the cool and charming guy they want to get with, and it seems like even though people who read KS are aware of this, they still get tricked by the very same façade.