In the last chapter Kyungsoo was thinking about how much he believed he emotionally hurt Donggook because of everything that happened. Because Kyungsoo sees Donggook as a pure, good-hearted person, Kyungsoo is blaming himself by saying he’ll never be happy again because of the pain he caused. When Kyungsoo met Yule again, he realised that Yule must be feeling the same way. Yule emotionally, physically, and sexually hurt Kyungsoo and Yule’s finally come to understand how much pain he’s caused to Kyungsoo, who’s also a pure and good-hearted person.
So in the end, Yule and Kyungsoo, who are blaming themselves for all the pain they’ve caused (Yule @ Kyungsoo and Kyungsoo @ Donggook), figured that they might as well stay and suffer together.
I’m interpreting the ending as Kyungsoo placing himself in Yule’s shoes?
“I hurt a good guy. I’ll never be happy again.”
Kyungsoo strongly believes he hurt Donggook, and Yule has become more self-aware that he hurt Kyungsoo.
I suspect the ending was “We both hurt a genuinely good person, and neither of us deserve happiness. Might as well suffer together.”
Christ guys...... this manwha was depressing from start to finish.