Why didn't he just castrate and kill him?

I'm trying to find a movie but I'm kind of hazy on the details. I think the trailer came out a year or 2 ago so either it's fairly recent or coming soon, but it had something to do with the protag being a doctor but also secretly a serial killer, or he was hunting a serial killer I can't remember. Pretty sure it was either Korean or Japanese, and had heavy I Saw the Devil vibes. Any guesses at what it might have been? I asked in /film/ as well but figured I'd try here as well.