Okay its a bit late but....Where is the part in this flashback where we get to know how he learned how to fight? we saw him do wrestling now we see him start murdering a group of gang members. How did he learn all this? I hope this flashback shows us the training he did.
Also Dongseok's friendship with Jinho is gonna be the death of him. he could've just ratted him out. but yeah why didnt he?