Why does Jun Mo shoot before Gi-Cheul does?

Most importantly – Jun Mo does this to spare Eui-Jeong. In his bitterness and despair, Gi-Cheul threatens to remain as a tormenting memory for Jun Mo and Eui-Jeong. In that last moment, Jun Mo perceives that the only remaining “good” outcome is to take the blame for the betrayal entirely on himself. We infer that Eui-Jeong is able mourn and let go of Gi-Cheul peacefully as she has left the necklace at his gravesite.

Secondly – Jun Mo resolves Gi-Cheul’s dilemma. Gi-Cheul has now decided to escape the gang and live a normal life. But Jun Mo explains there has to be a consequence for his sins, a price to satisfy justice. At the train-crossing, Gi-Cheul realises the price is too high: he has lost his dream, his crew, and his hyung (Seungho). In desperation, he places his hope in a future with Eui-Jeong on the run, but when that falls flat, he no longer has the courage to go on. Now he wants to die, even with the shame of suicide. But before he can do so, Jun Mo grants Gi-Cheul one last favour: an “honourable” death: a gangter in defiance, shot by a cop. by Vinh B