Say Yes (2001)

If you want to live, tell me I can kill your girl...

Young married couple Yoon-hee and Jung-hyun go on a road trip, and along the way they pick up a lone drifter, M. M turns out to be a violent and sadistic psychopath who terrorises the couple at every turn. After capturing Jung-hyun, Em gives him a choice: submit to torture or allow Em to kill Yoon-hee.