Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Cynical
If there's one thing Nina's tone as narrator conveys to us, it's heavy cynicism. Nina is world-weary, sarcastic, and doubtful about darn near everything, so her tone is all of those things, too.
After Casimir's death, for instance, Nina tries to reassure herself that she's not a typical vamp like Casimir was—you know, so she's not likely to end up like him. She tells herself: "I was different. I was active and empathic and dependable and involved. I wasn't anything like Casimir" (6.94). But since she's so cynical, and can't just leave it at that, she adds:
It's funny what lies you tell yourself when you're scared to death. (6.95)
Oops. This little statement undoes the positive thinking of the previous statement, further reinforcing the cynical, dark, and depressing lens through which we see everything thanks to Nina's narrative tone. Fun times, eh?