A Bittersweet Life (2005)

Kim Sun-woo is an enforcer and manager for a hotel owned by a cold, calculative crime boss, Kang who assigns Sun-woo to a simple errand while he is away on a business trip; to shadow his young mistress, Hee-soo, for fear that she may be cheating on him with a younger man with the mandate that he must kill them both if he discovers their affair.
Original title: 달콤한 인생 (2005)