@Supermacaquecool: Where do you get those labels from?
Mangafox and some other sites xd
However, I think they're accurate. Some of the horror genre's characteristics -according to Mike Chinn's book
Writing and Illustrating the Graphic Novel- are:
-The atmosphere: it has to create the panic sense, some even have fatalist and sarcastic climate. The decadent environments are a must.
Many of the current horror stories set a environment that is familiar to us to later twist it.
I think MN achieves this, while not perfectly. Let's take as an example the first arc (in which the third is killed). Yukiteru is the typical aloof weird kid that has no friends and is constantly bullied (well, Yukiteru is a bit of a extreme example...but I met a classmate that was very alike o_o). The story developes in the classroom, his home and the usual route he takes to go to the said places. This setting is familiar to us, so it's effective because we can relate.
Regarding the decadence, it's not that strong because let's say those places are very common and normal...I think that the decadence was more within the MC...Let's say that his way of living was decadent because he was miserable and trapped in a sort of fantasy world.
Now, they way the environment is twisted is with the presence of the serial killer. Suddenly those common and familiar places weren't safe anymore. There was some demon lurking...
-The Violence (either gore or not): One of things human value the most is their physical integrity and thus the violence (the deformation of the human body) is a topic that freaks the audience easily (or used to xd Lately the audience seems to be desensitized regarding violence...but that's another thing). Usually, as the violence shown become more realistic the greater it's the impact.
The fact that destroying the diary provokes the death of the owner helps to minimize the violence in MN, but let's say that the characters are very vulnerable and easy to kill (they can die devoured by dogs, with their throat slice by hand knifes, poisonous gas, gun shots....so it's realistic enough it my opinion.)
-The topics:The topics must appeal the fears and worries of the audience.
This probably the strongest point of MN. As enemies we have a serial killer (that is one of the most common horror fantasies of modern times), a terrorist (at least Minene looked like one in the second arc xd) and some other picturesque characters. Also it touches many sensitives topics such as murder out of benefit, rape, family disintegration, the weight of our decision and so many other stuff xd
So...I believe Mirai Nikki can be labeled as horror.