So Jinhyeong was the one who first proposed the idea of leaving the family behind you're stuck with, to build their own little family. He gave Song-Rik kind of hope.
Song-rik genuinely considered him as his real family, even considered dropping out of school to work extra jobs to fund Jinhyeong's education.
But when later Jinhyeong went back on his promise, specially when Song-rik was feeling too low, Rik must have felt too betrayed.
But again they're both children at this point, and Song-rik also doesn't have responsible adults around me to teach him better.
I knowww thats why i can't blame nor justify it
He never went back on his promise tho
They were still moving out but he just didn’t want to cut his father as he was feeling there was a chance for both him AND Rik to be accepted.
See, that's what I'm saying.. the promise was leaving their "shitty" family behind. He changed his mind after receiving affection from his father. He wanted both. He felt he could achieve both. While he was feeling that things can work out, Rik was feeling something else entirely, thanks to the glasses classmate. His inferiority complex, self pity or even hatred, grew furthermore. He wanted to run away at that very instant with Jinhyeong. He thought they were on the same page.
He clearly didn't expect Jinhyeong to start feeling otherwise, other than what they had discussed previously. So,, he did go back on his promise. Again, there is no need to defend anyone here. They're both kids.
In the future, we already know Jinhyeong is very clearly separated from his father and is dealing with various feelings and emotions. And sorting them out.
I GET SONGRIK BUT ITS HARDDDDDDDDD LEAVING THE PERSON WHO RAISED YOU ESPECIALLY WHEN THEY FINALLYYYYY ACCEPT YOU. THE GUILT.... ahhh cried for them, must've been a difficult situation