I know I shouldn't be, but because I became so interested in the idea based on my recent research into primitive war I am reading "less than human: Why We Demean, Enslave, and Exterminate Others"

It has an interesting slant:

In this book, I will argue that dehumanization is a joint creation of biology, culture, and the architecture of the human mind."
Dehumanization is a scourge, and has been so for millennia. It acts as a psychological lubricant, dissolving our inhibitions and inflaming our destructive passions. As such, it empowers us to perform acts that would, under other circumstances, be unthinkable.