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WARNING!!! MAJOR MAJOR SPOILERS!!!





This is the best explanation I can come up with:

Yeon-woo and Jun were witnesses of a curse in the black room. Jun forced these memories out of his mind because in most instances, people who experience a great amount of trauma will instinctively force the events out of their memory so they can live a peaceful life. Over time, Jun begins to remember the events of his childhood, which is what Yeon-woo wanted. I'm pretty sure (this could just be speculation) Yeon-woo invited Jun's friends over for the birthday party so that he could force the rest of Jun's memory to return (which worked lol). When Jun gained all of his memories back, he realized his connection to Yeon-woo. They are connected deeply, this is shown as them being literally fused together in their reflection near the end.
The story has ended here because Jun has nothing more to remember, he's fully accepted what happened in the past and present. When Jun remembered the explanation of "paradise" from the reflections of malice, he accepted it like it was normal. He smiles as he steps in the puddles of blood because he no longer has any sense of fear from the curse, only joy that he's fully a part of Yeon-Woo.

I hope this makes some sense?? (￣∇￣") I'm not used to explaining stuff like this, it's just my thoughts,,

YES it makes a lot of sense especially when you said “The story has ended here because Jun has nothing more to remember, he's fully accepted what happened in the past and present”