Ha! Your question caught me off guard. No hard proof. I just figure some shizz is about to go down, plus we haven't seen any cops get murdered yet (if I'm remembering correctly). Too many cops still alive so 1 might die or be badly injured soon.
We keep getting teased with the younger cop so maybe Kwak's death/disappearance/hospitalization will set him straight and he'll face off against Sangwoo? Dunno. Wish I had some details for you, but I also don't like SPOILING stuff for people. [THESE ARE JUST THOUGHTS] ╮( ̄▽ ̄)╭
Yeah, but Sangwoo's too far gone......again. I don't know what it is with those cops anyway ┑( ̄Д  ̄)┍ Maybe they need to brush up on training or investigative skills bcuz it obviously wasn't the first time a death/murder occurred that possibly linked Sangwoo. So, they should've kept an open mind after young cop freaked out. Sangwoo might not kill Kwak though, or Bum might do it........FOR LOVE.
OR, Sangwoo shifts blame onto Bum. Dunno
Did sangwoo kill a cop Σ(っ°Д °;)っ ╥﹏╥(⊙…⊙ )