i just got around to reading side 14 (cause i was holding myself back from reading it cause i didn't want payback to end) and i 100% agree with you
idk what i expected from the very last chapter but this bittersweet/sad note was not what i was expecting T^T
i thought it was gonna be more wholesome content with yoohan and jae not a circle back to his little brother
i mean, it kinda fits. yoohan started entered the entertainment industry to avenge his dead brother, to which he did, and he finally got closure from the man who murdered his brother and his feelings of regret. it's a fitting end for me. to me as well, him looking up in the sky and appreciating its beauty was an implication that he now enjoyed living, that he was not only surviving because of his karma.
you could say, he had "paid back" all his debts to his family, his regrets and his misdeeds.
Right after reading side 14, i went back to reread side 13, just gonna pretend that's where it ended cuz i dont want to end this manhwa in a bittersweet/sad note . I wished they put side 14 in the earlier side stories and ended with more fluff. Give them more happiness. Jay rubbing against yoohan like a cat in front of everyone was funny af