Killing Killer

The main character, Si-U Lee, lost his parents to a murderer ten years ago. Finally, he meets Jack, the one who killed his parents, and beheads him. “It wasn’t me who killed your parents.”
After learning that the real culprit is a "contractor" belonging to a group of killers using superpowers, Si-U Lee decides to hunt down the registrants with Jack’s help.
(Source: Naver, translated)