Well, that was an unpleasant mess.
It's only the first half of Mohiro Kitoh's original manga, mind you, which gets even nastier than this. Dark Horse removed pages from their English-language release and got Kitoh to redraw more of them, before eventually dropping the series after only seven of its twelve volumes. The German release is complete, but still edited. Even this off-putting anime adaptation flinches from the worst content.
(Mohiro Kitoh's next series after this, Bokurano: Ours, sounds equally anti-fun. It's about junior high school students who pilot life-draining giant mecha to save the Earth, but even the winning pilots die after each battle. It got an anime adaptation, but even the director disliked the source material.)
Anyway, Narutaru. It has a jolly title sequence and a nice, enthusiastic eleven-year-old heroine, Shiina. (She has her twelfth birthday in ep.4.) Shiina's normal. You'll like her. One day, she finds a star-shaped alien thing with shapeshifting abilities and unknown but limited sentience. She calls it Hoshimaru and takes it home, since it can do cool things like turn into a flying surfboard and let her play at being the Silver Surfer.
It can also grow spears and stab people through the chest.
Unfortunately, other people seem to have been finding similar things. Almost all of them are miserable, suicidal, evil, bullied and/or annoyed because they weren't the first in their peer group to kill some strangers. There are suggestions of incest, twice independently, between unrelated pairs of characters. There's a lot of morose dull negativity. There's not the slightest shred of happiness. You could probably make their heads explode by forcing them to smile. They become more likeable when they're releasing poison gas in a location where it'll drift into a nearby town and kill civilians, because anything's better than their grey, disinterested attitudes under normal circumstances.
I called this anime a mess because it's disjointed. It introduces a piece of trash who thinks the world's too complex and wants to start by killing five billion people. (My subtitles said "500 million", but no.) Then, later, we meet his friends, who become the main baddies for a few episodes and then get forgotten. They don't get killed. They don't run away. The story just gets bored with them. Oh, and these people kill probably hundreds of Japanese Self Defence Force soldiers, destroying planes, tanks and helicopters, followed by that poison gas leak into a nearby town... and this is immediately forgotten, with no consequences. It wasn't on the news. Life continues as usual for everyone. Akira remembers it, but otherwise I'd have assumed reality must have been rebooted.
The story then portrays shitty attitudes towards sex. Two girls who like boys get hurt by rejection. There's pregnancy. (Not all the schoolgirls are as young as Shiina, by the way, but we're only talking about the difference between "very very illegal" and "terrifying".)
Then, after that, a reasonably normal supporting character becomes the victim of bullying so extreme that I can only call it misery porn. Guess how well that turns out.
This probably hangs together better in the manga, but there's no chance of my ever reading that. I know how it ends, incidentally, and bloody hell.
Meanwhile, the art style can make it hard to tell the characters apart. Hairstyles can help, but good luck with the black-haired schoolgirls. (One of them makes things harder by cutting her hair.) This was a deliberate decision. The anime stuck with Kitoh's thin-limbed, angular character designs, perhaps reasoning this show had no chance of mainstream commercial success and so might as well push the boat out.
This show's problem isn't that it's dark, but that it's dreary and can feel pointless. I suppose there's some satisfaction to be gained from seeing shitty people get what they deserve in ep.11, but there are equally bad people who make the world worse for a few episodes and then get forgotten. (Note also the teacher in the final episode whose response to a confession of bullying is "don't worry about it" and barely changes his facial expression when a monster's killing his pupils in front of him. The latter's just cheap-arse animation, though.) The show's so bleak that it can get predictable. I liked Shiina, though.