Who is the devil in 'I Saw the Devil'?

Kim Jee-woon, in an interview:

There are a few different meanings for the title. Obviously the first would be that we see the devil in this serial-killer character and when we first come upon him, and second would be when we first thought it was just a serial-killer, we start seeing the descent of So-Hyun’s character in such desperate and extreme ways that we start seeing him turning into a devil. The third would be the audience members finding inside themselves the desire to see a more complete kind of revenge, and kind of wanting to watch this scene of revenge play out and take its course, and finding in that, in finding in a dark corner of their own selves, a devil inside there. Those are the three major ways you can take the title. So by taking note of these very raw, very basic desires and passions that are dwelling inside those dark corners of humanity, is where the title takes its inspiration fromeThe film is about a man who steeps to the level of a serial killer in order to punish him, thereby engaging in a rather vicious circle of punishment that doesn't really end well for anyone — in the end, Soo-hyeon is responsible for the very same loss of a family member that started the film.