There's so many ways you can portray this.
As Jun slowly starts to piece everything he forgot together, he soon starts to realize, Yeon-woo, and the house aren't the only ones strange. What truly lies behind the black door?
The black mirror, hidden in the black door are mirrors with a single TV that was used to play horror films. Why horror films in general? To me, horror films can make you feel scared and uneasy. It gives you that sense of fear and terror. The mirrors all around the room could be used to portray how you see yourself and how you view yourself. This can be seen with Jun's experience. He started to hear whispers and noises as he started staying and living in the house with Yeon-woo, just like how one would if they watched a horror film, you'd slowly start to develop nightmares. There were also scenes in the story where he kept looking in the mirror, choking himself, to relieve stress and to maintain himself, it's a way to take out his emotions - but just like so, mirrors are used for you to see yourself. At the end of the story, Malice was the true spark of everything happening, and how they had become "takers".
The Black Room can be portrayed as a place where you hide to when you feel afraid, scared, and fearful. The mirrors could have been seen as a way to REFLECT but also to see your actually intent and emotions - your reflection being portrayed as your " real thoughts " or " who you truly are deep down ". The horror films played could be portrayed as people who you thought you could uphold to but in the end they merely just drained you. (In the story, it was portrayed in many ways, such as parental abuse, betrayal, being used, never living up to expectations, etc.)
At the end of the story, the mirrors started to shatter, it could be seen as all the emotions and intrusive thoughts starting to come out and take action - you are filled with heavy emotions and ill intentions. And then poof, the people started to... evaporate.
People really are the problem and at the end of the day, people are just really fucking evil.
Black Mirror