I also reread now that everything makes sense.
Thru my reread I think Jingi is bipolar bc of ch 24. I think he keeps the childhood sa to himself bc he has intense ptsd from his uncle (ch 18) and his uncle is the one giving him what he wants in prison. Jingi's in prison bc he was able to reduce his brother's sentence from 20 yrs to 10 yrs. What I don’t get is how in the beginning he was sudicial in prison (ch 24) and how his uncle keeps all the tapes of it in his house (ch 51), to now, Jingi fighting for his life when there’s a hit on him from his uncle. I think Ch 57 insinuates that once he gets out of prison he’ll kill his uncles whole family?
I think it’s inferred that Heonjae assisted in his mom dismembering their dad. That or maybe he was part of a gang that dismembered people for body parts/organs for his debt before he dismembered his teachers legs idk. I think he’s more schizophrenic than ocd.
My theory is that Heonjae tries to kill Jingi. Myung il probaby stops or kills Heonjae instead for a longer sentence. Jingi gets out of prison and kills his uncles family and goes back to prison w/ Myung il as death row inmates. I hope all the kids get good endings at least
the mental illnesses you describe don't make sense he's not bipolar, hardcore traumatised with ptsd and anger issues yes but jingi is hypersexuai using his own trauma on others to cope but he's not bipolar. The same goes for heonjae that man is not schizophrenic but a violent sociopath there's difference but i agree with the last part and the op heonjae is probably going to kill his brother the story is working towards it idk if the parallels of their parents will be a factor but the attempted murder on jingi has already started everything

After rereading half of the chapters, I figured both siblings represent one of their parents. Jingi looks like his dad and got his anger issues too. We can even see him giving money to their mom but still hurting her, same thing Jingi does with MC. It was also mentioned that their dad had many problems but never expressed them (having debt). In comparison, Jingi never tells people about what his uncle did, not even to his brother.
As for Heonjae, he takes after his mom. He upholds a lot of mental pain without losing his cool, but once he does lose it, someone's most likely dying or getting severely injured. Same way his mom maintained their dad's abuse but killed him when she couldn't anymore. It's also mentioned both him and his mom started speaking late, could again confirm they both had autism since it's already obvious on Heonjae + severe OCD for him.
My deduction is that Heonjae might kill Jingi the same way their mom did with their dad