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Tldr, the flashback pretty much sums up that MC and ML were childhood friends of some sort, MC's twin died (maybe due to blood loss from having his tongue cut off), abduction of kids to use them for sexual exploitation. The part where kid!ML staring at the wall with the handkerchief was where the time he stole money for the sick twin only to come back and saw them gone (the time where this woman approached them).
The warnings helped me piece the clue together and just the implications left me feeling disgusted.
I cannot... My heart hurts for these children. The fact this story reflects the reality is just too upsetting....
It doesn't help that this story took place in Korea where they take these crimes way too lightly for my liking. I hate it how some actual criminals are still walking free after commiting unforgivable crimes... I hate it.