I think the "evil spirits" living in the mirrors represent their trauma and resentment towards the world. They're not the only people with trauma and the "power" to kill people using the resentment energy. That's why there's all these evaporation cases all over the world.
The special thing about our two main guys is that they have shared trauma so them as a whole can control the resentment energy. If they are separated, they aren't complete and they can't willingly control the mirror hands.
That's what I think anyway.
this has quickly become my new favorite story. the pacing is amazing, the flashbacks don't feel dragged out or expository but rather they give you the feels so effectively. i cried when mojil passed and i can feel the hopelessness soongap feels. he's been through so, so much. glad he's got bongchun in his life now, this gentle giant with a giant heart. im in love with this manhwa already