Horror
Chances are the copy of Carrie you have features a girl covered in blood. Um, yikes. That's not something you see on family-friendly material, like your Wheaties box. (If you're eating cereal that does have blood on the box, please do not invite us over for breakfast.)
King's main goal, we think, is to unsettle the reader. He constantly foreshadows the disastrous climax of Carrie, which leaves us on the edge of our seats the whole book.
And he doesn't shy away from sex, violence, or gore. Despite being named Carrie, the book is less a coming-of-age story about a young girl, and more about the horrific events that transpire when that girl realizes her own power… but can't, or won't, control it.