
Here are some new things I think we just learned about Sangwoo:
1. Does NOT like old people
2. He still thinks about his murder victims
3. Has a loose mouth when drinkin
4. Has a complex anout getting old
Now with the whole "I win" comment at the end. I think it wasn't about having sex with Bum that made him puke. He has been drinking and not only that but remember in season 1 with the old man at the gay bar? The old dude had Sangwoo suck his dick at that time, and Sangwoo really held that to him. During and after the murder he keeped talking about it. I think there is something more to that also the fact that sucking could be seen as a submissive role in sex. If he was grossed out about having sex with Bum I think the author would have shown that. He actually showed that he liked it given that (in the beginning of season 3) they already did it twice. And giving him a handjob while Bum was driving (for the first time!).
So I think him saying "I win" is him addressing the fact that:
1. He didn't puke in front of Bum <- that would be seen as weak
2. He didn't go submissive and suck Bum off
3. At the end still showed that he was still in control
Another weird thing I've been noticing is that he keeps bringing up Bum's age. (And what is his age? Bum said he saw Sangwoo when he went to the same university and were in the same troop in the military) Like I said before he doesn't seem to care for old people (especially men) probably has to do with his dad. Bum (to Sangwoo's eyes) is old and a man.
Just my thoughts ┗( T﹏T )┛

I thought "i win" meant he was better than bums uncle, since sangwoo brought him up a lot, especially when he brought up the fact that bum only drank with his uncle, sangwoo wanted to overwrite all those bad memories. But you make a very interesting point. I never thought about the whole submissive thing and it totally makes sense too
"When are you going to breakup with that young lady?"
"When are you going to stop being creepy and asking me personal questions in the dark?"
this made my day lmao