Evidently, Seo-In doesn't know that Yeongwoon got beat up, and thought he just left—"started over" without saying goodbye with "no regrets" (just like with the video game Yeongwoon was playing when they were younger).
To Seo-In, Yeongwoon was his only "escape" from his brother. Literally his only hope. So for Yeongwoon to just leave him with no regrets, that's like the biggest betrayal to him, which is why he wanted to see Yeongwoon suffer—to hurt him and leave him just like Yeongwoon did to him.
But when they meet again, that's not enough. He made Yeongwoon suffer, yes, but all of those feelings he had for Yeongwoon when they were younger were also coming back to him, and he realized he couldn't let him go.
I love your theories!!
Your second point makes a lot of sense!! I think this might be the closest to the truth! The only thing I'm wondering about is why it was Seo-in who was grabbing Jaewon by his shirt.
Was he about to (idk) punch him and then Jaewon suddenly leaned forward to make it look like Seo-in initiated the kiss?
Because I doubt Seo-in would want to kiss Jaewon
Omg i have so many theories.
1. The day Yeongwoon told Seo-In "how much longer do we have to keep hanging out? I'm sick of it", we already know that this is the last time Seo-In saw Yeongwoon before he left. I think this is the same day the "kiss" between Jaewon and Seo-In happened. Which leads me to my next point:
2. Yeongwoon was the only person that Jaewon could never "beat", so the only way for Jaewon to beat him was through Seo-In. So i think the "kiss" was Jaewon's plan to get back at Yeongwoon.
-> Jaewon said something along the lines of "don't look at me the way yeongwoon does", meaning Yeongwoon KNOWS he can beat Jaewon, so he looks at him like Jaewon is not a big deal to him, he's nothing. But then he sees the way Yeongwoon looks at Seo-In, and it's totally different. Because Seo-In IS a big deal to Yeongwoon. So he plans for Yeongwoon to see the "kiss", because he knows it's the only way to get back at him for always beating him, and it's also a way to get back at Seo-In for being the only one Yeongwoon takes seriously.
3. In the previous season, Seo-In's aunt (or someone) mentioned that Yeongwoon was always coming over, and was always playing with Seo-In AND Jaewon. I don't think Yeongwoon knew about the hatred between the two boys - i think he thought he was friends with both of them.
4. Seo-In always acted as Jaewon's "shadow", beating people up for him, etc. AND during the scene where Yeongwoon is getting beat up, one of the guys says something like "Seo-In only hung out with you because (...)". So it wouldn't be too far-fetched if maybe Yeongwoon thinks that Seo-In knew the beating was happening—that Jaewon was maybe doing it ON BEHALF of Seo-In for the feelings Yeongwoon caught for Seo-In. Which is maybe why Yeongwoon "can never understand" Seo-In—because Seo-In shows him a side of himself that doesn't match with what Yeongwoon went through on Seo-In's behalf.
5. As I mentioned in another post, I think the mom thinks SEO-IN was behind Yeongwoon getting beat up—that she thinks Seo-In told people to go beat him up because Yeongwoon "had feelings" for him and thought it was "disgusting" -> that's why when she sees the scandal with Seo-In and other men, she says "Seo-In would never do that" and she's SHOCKED to see them together at the hospital. WE know Seo-In is not behind the beating, but I don't think anyone else knows the truth, which was why the random kid with freckles told Seo-In that he "has no right" to look for Yeongwoon.
I'LL BE BACK WITH MORE THEORIES SOON AND CAN'T WAIT TO SEE HOW IT ALL UNFOLDS.