Fan Casting Kim Bong-gu in Scales: Mermaids are Real - The King 2 Hearts (2012)

The series antagonist, a magician by day, an arms dealer, a terrorist mastermind, and an assassin by night. He decides to assassinate the South Korean king after peace talks are established between both sides of 38th Parallel which compromised the interest of his weapons business. He also has an even darker agenda waiting to be revealed but was executed following his crimes and only appeared in a flashback.