I read the novel and the extra chapters retell the story from his pov. He kept it so that he would not be tempted to act like nothing happened and be with Sejin without telling the truth first. It was a reminder of how he broke sejin and therefore doesn't have the right to keep him
ahhh do you still have the link for the novel?
I read it on Spanish but here u go!!!
https://www.wattpad.com/story/368010666?utm_source=android&utm_medium=link&utm_content=share_reading&wp_page=reading&wp_uname=LoloLala893
I wonder if ML ever goes back to the symbolism the gun was suppose to represent and laughs at his past self of how idiotic he had been. His future self still kept the gun of course know locked and with no bullets, but why not throw the gun all together? Those it serve as a reminder of what he has done and his goals and his second life