The Gangster, the Cop, the Devil (2019)

In Cheonan, a serial murder case takes place, but the case becomes more and more of a mystery. Only Tae-suk, a cop, immediately senses that all the cases are by the same killer and starts investigation. One day, Dong-soo, a gang boss, got attacked by a suspicious man, and Tae-suk realizes that this man is the serial killer, K. Now, Dong-soo becomes the only witness and a survived victim from K.