Being 13 episodes long, each episodes being less than 25 minutes long, the length of the episodes is certainly not a problem, and works to set the ambience of the anime. However, it is the themes of the anime which make it a difficult watch. It might not be an artistic presentation, but it definitely is a thematic fright, because it talks about the "darkness among men". Within its themes are: cannibalism, necrophilia, murder, and serial killing among the handful few obvious themes. Each of these themes is the integral part of the episode, and are all small steps towards an overarcing story which is concluded in episode 12 and 13.
One criticism which I have in terms of sound, is the theme song "the flame" which opens the anime. It isn't disconcerting, but it isn't serving a significant purpose towards building the anime either. Furthermore, in my opinion, it doesn't add to the opening of the anime visually (which is really good, and is undermined because of the theme song). I don't count art as requiring criticism because in my opinion, it is aged but it has aged well - and with respect to the anime, it adds an overall sombre tone to the story, which definitely works in its favour.
All in all, here's my two cents on it: If you're really into understanding how dark can a human heart be, watch this anime. Its a decent work to say the very least.
