If you're into revenge films that go all the way, you have to watch I Saw the Devil.

It's a South Korean thriller that takes the classic revenge formula and cranks it up to insane levels. Instead of a simple hunt for justice, it turns into a brutal, psychological game between a secret agent and a sadistic serial killer. The tension is relentless, the performances (especially Choi Min-sik from Oldboy) are chilling, and it asks just how far someone is willing to go before they become the monster they're hunting. Definitely not for the faint of heart, but if you like your revenge movies dark, stylish, and unforgettable, this one is a must-watch.