Terrific movie. It's my favorite Korean thriller together with "Memories of murder". Very different films, though. "Memories" is a relatively subtle procedural with a lot of quirks because of the setting. Reminds me of an old Spanish film, "El cebo", which Sean Penn remade as "The Pledge".
"I saw the Devil", on the other hand, is off the rails from scene #1.
If you like Memories of Murder, you'll be happy to know that it's going on Criterion eventually. Which I'm very happy with because I can't find any Blu-rays for that movie at reasonable prices.