Mail or sleepless 2017

You're referring to the 2017 South Korean psychological thriller film "I Saw the Devil" (also known as "Mail" or "Sleepless").

"I Saw the Devil" is a 2017 South Korean film directed by Kim Jee-woon, starring Lee Byung-hun and Choi Min-sik. The movie is a remake of the 2006 South Korean film "I Saw the Devil" (also directed by Kim Jee-woon), which was a critical and commercial success.

The film tells the story of a secret agent named Jang Kyung-chul (played by Lee Byung-hun), who is tasked with tracking down a serial killer named Oh Ji-hun (played by Choi Min-sik). As the investigation unfolds, Jang becomes increasingly obsessed with catching the killer, leading to a cat-and-mouse game between the two.

The film received positive reviews from critics, with many praising the performances of the lead actors, the tense and suspenseful atmosphere, and the thought-provoking themes. If you enjoy psychological thrillers with complex characters and moral ambiguity, you might enjoy "I Saw the Devil".