In the mature version, Yuri is giving head and then he swallowed even though Seon Woo asks him him to spit it out ... Boy has so many issues but still gotta give him credit for giving good head y'al
Well Won you got what you wanted but at what price? The loss of your lover's sanity? To see him be an unhappy miserable f&ck? Well I hope you're happy.
Pretty sure Won did the same thing to his brother, isolated him from others, manipulated the relationship, hurt Jinseo as well. Which explains the memory of his brother's expression when he got hurt (likely because of Won)
so he probably committed suicide because he couldn't take it anymore with Won.