When Angel saw the abyss-like black hole appear within the mirror, he understood that something had set its sights on him.
Earlier, Black Earl had entered the Stairway of the Hanging Prison and seen this mirror as well. He had even left a mark on it, yet nothing had happened to him. Why had something gone wrong when it came to Angel?
Comparing the differences between before and after they entered, the answer was actually obvious. There was one and only one reason: that oil painting.
Angel had destroyed the painting and taken the round piece of wood from within. That had clearly provoked some existence lurking inside the mirror. The moment Angel touched the mirror again, it immediately noticed him.
The pitch-black abyss displayed in the dressing mirror must have been that being's doing as well.
Its purpose was nothing more than to "force" or "lure" Angel into the mirror. As for what would happen after he entered, it certainly would not be anything good.
Therefore, the moment Angel saw that abyssal passage in the mirror, he chose to retreat, while sealing the mirror's surface.
—Just as Jon often said: a gentleman does not stand beneath a crumbling wall. Angel understood the principle of "no trouble, no death," so how could he recklessly wipe the mirror directly?
Before blowing away the dust, he had already made preparations and sealed the mirror's surface.
Sealing a mirror involved knowledge of space and mirror images, which happened to be the subject Angel had been studying recently. To research it, he had repeatedly entered and exited Mirror Wraith's Mirror Space. Thus, sealing a mirror was no difficult task for him.
A mirror was the interface linking two different "spaces," and it was at its most fragile there. Once sealed, it was very difficult