As The Shadow says, "Who knows what evil lies within the hearts of men?"
This is an easy question compared to: "Who knows what evil lies within the hearts of vampires?"
If this quote were be on the back of Interview with the Vampire, it could sum up all the internal conflict that lies within the book—and within its eternally tormented characters. The battle between good and evil overlaps with themes of sin and religion, but all these questions boil down to the same thing: are vampires inherently evil, or aren't they? And on top of that, does evil even exist?
Interview with the Vampire isn't a novel of good vs. evil in the sense that you know it. Townspeople aren't battling evil vampires in castles. These vampires are battling the evil within themselves.
Questions About Good vs. Evil
Are vampires evil by nature? Are people?
Is Claudia evil? If so, is it because she's a vampire, or is it because of how she was raised? If she were not a vampire, would she be good?
Does Louis become evil at the end of the novel by slaughtering the vampires in the Théâtre des Vampires?
Is there any way for Lestat to redeem himself?
Chew on This
Vampire nature is inherently evil. At the beginning of the book, Lestat represents vampire nature—he kills without remorse and enjoys it—while Louis represents its opposite. Louis and Lestat switch places at the end; Louis becomes evil when he chooses to give in to his anger and slaughter the Parisian vampires.
A vampire isn't born good or evil: it's all about how they're raised. Claudia, raised by Louis (good) and Lestat (evil) becomes a volatile combination of both.