The Way of All Flesh by Tim Waggoner

In a world where zombies battle the living, which is more terrifying?
David is trapped in a nightmarish version of his hometown, pursued by crimson-eyed demons and insane cannibals, with no idea how he got there. At every turn he's taunted by a mysterious youth named Simon who knows far more than he lets on.
David's sister, Kate, fights for survival in a word decimated by flesh-eating zombies-and her brother's one of them. She's determined to put a bullet in David's brain to set him free.
Nicholas Kemp is a human monster, a born killer. But in a world ruled by the living dead, he's no longer the most feared predator, and he'll do whatever it takes to become that again. He plans to start by killing Kate.