I think he only kissed him becuase the kid was at absolute rock bottom, and he was trying to save the one thing that actually cared about him. For him, the MC is the only thing he cares about and doesn't want to lose.
For the MC, he has two people he loves very, very differently. The ML, who he loves and sees as someone that is a friend/lover, who he feels guilty about breaking away from, and gangster daddy, who he sees as a friend/big brother who he knows will protector him and stick round, even when he has seen him at his worse.
So for thr MC, he has always been fsmily he doesn't want to lose. For thr gangster, the MC is his only precious thing.
Classic one sided love.

Another thought is bothering me.
If he kissed him then, in a moment of extreme fragility and loneliness, in the need to be accepted and loved... what went wrong that they're not together as a couple, but still in this questionable parent-child relationship?
It's not that the guy was faithful to his first love, the blond guy (since he had a girlfriend), it's not that he wasn't bisexual, and it's not that he wasn't deeply emotionally attached to his hyung.
Well, maybe there was a sense of incestuousness to it all, and the guy was just really not his type. I don't know. It's bothering me now.