Umm, I don't know if what I'm gonna write will be considered spoilers, so I'll just spoiler-tag it to be safe.
click to show spoilers
Regarding the brotherly love, I thought it was an interesting aspect to develop. When watching the series, I was massively unsatisfied by GJ's lack of reaction to YK's admission of the murders. I was expecting GJ to at least be angry with YK and maybe pummel YK a bit or maybe disown him as a brother. How does GJ 'have the face to see' his teachers' spirits when he just told their cruel murderer scot-free without even a scolding?! But I guess for GJ it's hard to pick between his teachers and his brother. For such a loyal guy like GJ, YK is like a blood-related brother. Even if GJ never met his teachers, or never met YK, they'd still be brothers. And he promised his mom to always protect YK. So in a way, this is protecting YK. This probably doesn't make sense, but hey, maybe that's just the way GJ's mind works.
I mean, what will your reaction be if your sibling murdered your parents? Compare that to your spouse murdering your parents? What a horrible thought. But I'm thinking people will be more willing to put their spouse to jail. While if it's the sibling, it'll be more like "OMG, WTH do I do?!?!" kind of reaction.
And on his part, I thought YK said at that standoff that he did care about GJ, maybe more that GJ cared about him. He came to that conclusion because he was very angry/hurt/upset at GJ, and he heard that anger/hurt/upset is because of caring about that person. It may be weird logic and all, but that's his thinking. And afterwards, when repenting at his parents' grave, he did say he was thankful for GJ who made him realize the error of his ways.