She discusses her fears a lot. She's afraid of getting lost in the kelp forest, losing her sapience, and going completely feral. She's afraid the "energy form" will corrupt her soul. She's afraid of herself, because she's starting to enjoy killing ancient beings. The plot of the game involves a number of gods committing genocide, wars between races, and the lust for power leading to deep corruption.
I'd say 8 or so is the youngest that could understand it, and even then, I'd play it WITH them, and discuss the story as you go along. It covers some pretty deep stuff. There are intestines growing in the cathedral and the pulsing blobs of flesh... but in my experience, kids can handle that sort of thing pretty well (YMMV). What will probably cause problems is the fact that your avatar character is worried about becoming evil.
Kids like violence, to a certain degree... so long as its good triumphing over evil. Moral ambiguity and heroes who aren't so heroic? That's what gets scary.