The Waking Dark by Robin Wasserman
One day in Oleander, KS, a handful of people suddenly kill everyone around them and then commit suicide. Then, about a year later, a tornado rips through the town. After that, things start to get really bad.
The Waking Dark starts out gruesome but instead of sticking with shock value Wasserman layers on uneasiness and terror as the book progresses. Everyone has the potential to do something evil, including the protagonists, and this will pull you through the story even as you wish you could cover your eyes at some of the more terrifying parts. Read this if you want to see the darkness of humanity exposed.