I Saw The Devil (2010, Korea).
Watched this at the cinema and rewatched it many times since. A masterclass in filmmaking.
Starts off as a revenge story…gradually becomes darker: Obsession, hatred and psychological cost of vengeance. Choi Min-Sik (Oldboy) delivers one of the most terrifying serial killer performance. Lee Byung-Hun brilliantly portrays a man driven by grief and rage. The movie has extraordinary tension, technical excellence and moral complexity. And it is absolutely unsettling and brutal.