Dehumanization refers to the act of seeing victims as nonhumans. Dehumanization lowers inhibitions against aggressive actions and makes continued aggression easier and more likely
Other /More definition:
Dehumanization is defined as the process of perceiving members of a group as sub-human or inferior to members of one’s own group which allows people to inflict pain and suffering on the group without worrying about the morality of their behavior. Moreover, it is the belief that other individuals or entire groups of individuals lack the qualities thought to distinguish human beings from other animals and such Dehumanization serves to rationalize the extremely negative treatment usually afforded to members of other groups.