The shounen demographic at large focuses on power fantasies and more simplistic, idealized characters and values and in the case of battle shounen, more of a focus on action (shounen jump's motto is, after all, "friendship, effort, and victory"). The seinen demographic, however, tends to have a far more cynical outlook on life, unafraid to show the raw ugliness of human beings and a higher focus on the psychology and flaws of the characters, and tends to be a bit TOO liberal with its narrative freedom, including a lot of gore and sexual content (though of course many of them still remain great stories).
All of this is consistent with Himizu's artistic goals; it is a story about ugly, weak people. The main character despises others with ambitions and dreams and hides this under speech about wanting normalcy. His closest friend is a bucktoothed spineless weasel who skips class and is a pickpocket. One of their initial acquaintances is a fat slob who wants to be a mangaka because of some meaningless dream he had the other night. The main character's love interest is a plain-looking, bizarre girl. About the only well-adjusted and respectable character you see is the mangaka, who actually gets published and has his ambitions realized.
This is all fine, as it's been repeatedly shown that a character's moral fiber has nothing to do with how good of a character they are, and in fact many times horrible people will be far compelling than a straightforward good guy.
The main issue with Himizu is that...it isn't horrible, but doesn't EXCEL in anything it really does, barring one or two minor things. It tries to have the psychological storyline with pathetic, weak characters common to seinen manga, but while most of them aren't really BAD, they aren't really that great either. Besides being weak and terrible most of them don't have that much in the way of psychological complexity, excluding the main character who is by far the best character in the manga. Sometimes having simple characters can be fine, but here the characters, while "realistic" to a certain point, just don't have as much going for them as I would like.
The storyline also spends a lot of time focusing on these other characters, and while these subplots are never really BORING, they also distract from the main conflict that the MC is going through since the characters in them just aren't as interesting. The storyline left me...confused at what it was trying to achieve exactly?
The manga's art isn't bad. There are a few moments of genuine greatness, like the representation of the "demon" that haunts the MC's mind, which is very effective and disturbing, but otherwise, it's mostly just ok.
Overall, besides the confused story structure, there isn't much WRONG with Himizu, per se, but it just feels very average in its execution of the typical elements of seinen manga. I have seen many others which have done what Himizu is trying to do in a much more interesting and frankly better way, like Homunculus or Ressentiment. I don't regret reading it by any means, but it is a story that I would have probably liked much more like 5 years ago when I hadn't read as much manga as I have now.
