Jin was actually the first to die. The rest of them are grieving but in the wrong way by committing crimes, getting drunk. I personally think they all went insane after jin's death. Looking at the part where v and rapmon spare paint the wall, it's a reenactment of what they did to jin. Where they pushed jimin into the water and started drowning him, it's like how jin killed himself by drowning which explains the water stuff in the prologue. Jhope overdose in order to try and kill himself as well because he can't stand not having jin around. Junkook is the only one who has accepted jin's death. He punches minyounki because minyunki has been drinking to try to numb the pain of losing jin, but starts getting violent. I think the reason why they were all acting like this because jin had always been some kind of protection towards them, like the way he saved them in the tunnel. Followed by jungkook 's smile, it seems that jungkook has accepted jin's death but knows that he's still there for them.