I Saw the Devil
Recommended by my friend @Pixelking251, who said this is his favorite horror film, and boy does he know whats up!
This movie is awesome, very heavy but its a frenzied up adventure about violence and revenge that ends in a huge void. Despite the rather repetitive structure, in no moment it made me bored, it has two very enticing main characters and a purpose for all of those.
Good stuff.