The Plot (spoilers)
Up until the last 5 minutes it was a reasonable movie (not great, but passable, and the body on that Eve... well done, God). But then Kang turned out to be a real alien? WTF?
I wouldn't have a problem with that if the movie had developed more the reasons why Byung-Gu believed in this alien conspiracy, but as it is, it would make a lot more sense if the story was nothing more than a guy killing people who'd done bad things to him, using the alien thing as an excuse.
Some movie twists are an unexpected surprise and add levels of entertainment to an already good story ("Usual Suspects", "Fight Club" and "The sixth sense" gave me goosebumps, for example). But in this case, the twist (Kang being an alien) doesn't add anything. On the contrary, it damages the already weak plot.
IMHO, this movie would have been a lot better had it had less torture (which are not nearly as gory as some reviews will make you think) and more plot development.