Every single victim killed by Lee Tang—from his first victim to the "blind" girl Yeo-ok and high school bullies—was secretly guilty of murder, rape, or child abuse. Tang never gets caught because nature and luck destroy his evidence: flies cover security camera lenses, rain washes away footprints, stray dogs eat murder weapons, and fires destroy crime scenes.
A major plot twist reveals that Detective Jang Nan-gam’s father was not an honorable cop, but a corrupt drug smuggler who framed Song Chon's innocent mother, driving her to suicide. Roh Bin deliberately takes the blame for all of Tang’s murders and dies in a final shootout, leaving behind evidence pointing to himself and Song Chon as the killer duo.
In the final episode, Jang Nan-gam corners Tang in a burning factory after Song Chon is killed. Knowing his father’s true nature and the justice system's failure, Nan-gam lets Tang live and reports that Roh Bin and Song Chon were responsible. Tang escapes to the Philippines, gets deported back to Korea for illegal immigration, and learns his name was completely cleared. In the final scene, as Tang walks down a Korean street, his neck hair bristles upon passing a stranger—confirming that his paradoxical journey as a killer of evil will never end.