The MC probably trued to act tough a 5, and help the ML get over his fears but instead they saw the monsters in the mirror. Since mirrors are known as portals, it's safe to assume that this may also be a euphemism for split personality due to extreme childhood trauma. So one of the two was left to block his memories alone to cope, while the other was probably subjected to it by his aunt and uncle. 

By the time ML moved to America he'd learnt to cope with the darkness and embrace it, meaning when his aunt and uncle finally pis*** him off too far he took em out, or maybe when he was old enough to go back to Korea. The reflections probably do the evaporating, but idk how yet. And from the speech bubbles on the news, it's been going on for centuries.

It can't be purely a psychological break and cognative dissonance, but something supernatural at play. Something sinister that can use reflections at will, and thrives in the darkness. I suspect, the devil. 

The children made a deal with the devil, allowing for the blood pact between them and the routine sacrifices. Because the ML can control who it targets, hence why his critics keep going missing, and children in distress are easy sols to trade with. 
They made a deal with the devil. But hey