I suppose anyone can try to intrepret a manga in whichever way they like, but as far as I'm concerned I think Narutaru is a very pessimistic work. A speck of hope can be seen in the ending if you really want to, but it's quite neglectable. Speaking of the ending, it's probably the weakest part of the story and it is quite rushed; I don't know if the author wanted it that way or the manga was axed near the end. Nonetheless, it still delivers the message rather clearly, and the rushness even helps in making you feel the ephemerality of the characters' lives.
The premise is a girl, Shiina, finding a mysterious star-shaped creature (which she names Hoshimaru) during a vacation on an island where her grandparents live. Hoshimaru befriends her and, since he is able to fly, Shiina has a blast spending time with him, riding him to soar the skies. From here things escalate quickly and Shiina is attacked on a plane on her way home from the island. As the story goes on it becomes clearer and clearer that having power isn't exactly games and fun.
The author seems to have interesting opinions, like the way Shiina's mother critices the education system for putting false hopes in people's hearts. The intentions of Naozumi (who is pretty much the "antagonist" of the story, if there is any) are quite simple but surprising nonetheless. His character isn't very well explored, but you can tell that he dislikes the hypocrisy humans show in their belief that they're special and above all other living creatures (he goes fishing with Akira and Shiina with the purpose of letting Akira experience the joy of toying with life, and then he asks her what's the big difference in having fun playing with a fish's life or with a human's). At first he apparently wanted to create a simplistic utopian society according to his standards, though ultimately he ends up acting as a mere destroyer of all things. Naozumi can be brushed off as yet another emo-kid, but under the teenage angst you find rather believable opinions, dangerous opinions because he raises problems no one is willing to tackle, thinking that they will solve themselves or someone else will take care of it. The precariousness of present day society and its radical unfairness, plus the way everyone pretends that all is fine: this is ultimately what makes Naozumi hate the world and try to rebuild it from the ground up. But he doesn't wish to be one of those who will recreate it: he identifies his role solely in its destruction. He doesn't make excuses even for himself, and the method he chooses to kill himself (death by starvation) shows his contempt for the very foundation of life (you've got to destroy other lives in order to maintain your own). This implies that he doesn't hold much hope for the new world neither, but he doesn't judge since it isn't his role to recreate it; he just wants to leave the scene.
With Naozumi being the destroyer, Tsurumaru on the other hand is a creative principle. He goes around knocking up women as if nothing were, and he even has the balls to take responsibility nonetheless as he offers to pay for all of the childhren's expenses (with money he earns on his own). Tsurumaru, with his "free-love" and lack of monogamy, loves the species but he fails at loving the individual. This is probably what he learns from Norio's death and what he regrets when crying in front of one of his grilfriends.
As for Shiina, she is just... empty. In the sense that she holds to an empty and groundless optimism. It's very cruel the way the author mocks the cheerfulness of children, but albeit a sad turth it is a truth nonetheless. Shiina fails to see reality (or what the author deems to be relaity, though I must say that I agree with him for the most part) and she will learn the hard way how the world truly works. As Hiroko tells her, she can't save everyone with kindness. Shiina's naive idea that people can supposedly always reach an understanding and work things out without violence will be oftentimes shaken throughout the story. She manages to let out killing intent for strangers (especially the one that killed her father's friend) but she fails at strangling Hiroko when she threatens to kill Shiina's father, and Hoshimaru has to take it upon himself to finish the job. In her unfairness Shiina will abandon Hoshimaru for several months after he kills Hiroko, though he only did it to save Shiina's father. In her self-righteousness she doesn't want to justify Hoshimaru's actions, though ultimately when she will need his power again she will have no choice but to accept Hoshimaru's help again, as always. And she will even apologize to him. More than accepting and elaborate Hiroko's death, Shiina seems more willing to forget about it and try to pretend it never happened, as she somewhat keeps holding onto her ideals even though the facts proved her wrong. The triumph of hope over experience, and it will turn out to be a groundless hope. Though truthfully from the cheerful and somewhat honest child she used to be, she will become more and more of a facade, crippled inside by fear but still trying to deceive herself and other people into thinking that everything is alright.
After her father's death she is on the verge of breaking almost completely, and she regains her composure only for a short while, but as all her loved ones are quickly killed in the aftermath of the nuclear winter brought by Naozumi, Shiina wishes for the annihilitation of mankind, and that wish is granted. Ultimately she contemplates the shambles of her life, visiting once familiar places now reduced to ruins. Pregnant with Tsurumaru's child, she smokes; an emblematic show of the disrespect she now feels for life. At the conclusion, Shiina's daughter and Kuri's son are the last human beings on Earth, playing on a desolated beach, and as the story ends, they kiss. The new Adam and Eve. A new beginning. Will it lead to a better outcome, this time?
