I Saw The Devil was the better movie.
Be careful when you fight the monsters, lest you become one.
The Man From Nowhere is no joke, but it just lacked that extra layer. The protagonist is an anti-hero, but you don't get that sense that he's "become a monster while chasing one" like you do in ISTD. His character is just a little less interesting.
Also give
The Chaser a try. Another Korean detective/revenge type movie. Ex-Cop becomes a pimp and has to track down the person that killed his whore. It's also on netflix.
These Korean revenge/detective/film-noir movies have been
killing it over the last decade. It's a shame that black folks can't get their shyt together and start a mini film revolution like they have.