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I believe Yeon is “possessed” and Joon is “oppressed” by the demonic entities in the mirror. Yeon is doing the killing, not Joon (that includes killing the person on the street in front of Joon and in the theater). The reason why evaporations are happening all over is because Yeon has traveled all over for school/book signings/interviews and killed people during his travels.
All of it began when they were 5 years old and released whatever evil was in the room/mirror together after Joon suggested Yeon’s parents be murdered. The evil spirits in the mirror fulfilled their wish and has been possessing Yeon and tormenting Joon ever since.