Description can be your best friend, but overdo it, and the reader will want to go to sleep. I've read many a fanfiction like that. Sometimes, less description is more. Take the movie "Jaws" for example. That was scary mainly for how little you saw the shark. Granted, it's not a book, but you should get the idea. It would keep the reader wondering what did it and such.
Sometimes, not even the story, but the idea of such a thing happening is what's scary. This is the reason Stephen King's book "Christine" and the movie he directed, "Maximum Overdrive" were scary. If machines came to life and started going on homicidal rampages, most people wouldn't fare much better than the people in "Maximum Overdrive". As for Christine, to point it out, compare it to "Cujo". When a dog gets rabies, you shoot it. What about when your car comes to life and starts killing people, then has your best friends in it's sights?