A Bittersweet Life (2005)

Kim Sun-woo is a manager for a hotel and an enforcer for the hotel's owner: a cold, calculating crime boss. Sun-woo is assigned to a seemingly simple errand while his boss is on a business trip. He's to shadow his boss's young mistress, Heesoo, whom the boss suspects may be cheating with another, younger man. Sun-woo is mandated to kill them both if he discovers an affair, but he decides to spare their lives... at his own expense