I think that this story could claim elements of the thriller, fairy tale and horror genres and rightly so. The author has masterfully done this by using the third person point of view - (spoiler alert) the mirror.
Read the story now
Personally, I'm not drawn to graphic detail. However just as graphic language and sex have their place when they are relevant to the story, they must be included. So it is with "The Mirror". The descriptions of the crimes committed are not gratuitous, but entirely necessary to portray the evil, that I assume, lives within the mirror.
The only criticism I would offer is to be aware that the POV doesn't slip, as could happen in this type of story.