Book in the zombie genre about a teen and his older brother who are paid to hunt down people's turned family and friends and put them out of there misery.

I read this book around a few years after hunger games came out. There were bandits who were chaining huge numbers of dead to trees so they could find and kill people's family more effectively. There was a section about a feral girl they meet and how she was surviving. The book ends with them seeing a Plane pass over head setting up for a sequel but I forgot about the book and now I'm curious what happens.