Nearly every group conflict throughout history has involved dehumanization of the "enemy." Recent research suggests that this process is related to a fear of death.
University of Padova (Italy) psychologist Jeroen Vaes, myself and Jamie Goldenberg of the University of South Florida, recently conducted three studies in which the role of death awareness on humanization of in-groups and out-groups was tested.
In these studies, participants were randomly assigned to write about their own death, or an aversive control topic (such as pain or failure). They then rated their own group and an out-group (e.g., Italians rated Italians and Japanese people; Americans rated Americans and British people) on a wide range of traits. Next, participants rated these same traits on the extent to which they are "unique to humans (as opposed to animals.") The correlation between how unique to humans each trait was perceived to be and the perceived typicality of the group for each trait was the dependent variable.
Basically, in every study, when reminded of death, participants perceived their own group as more uniquely human. These results held even when controlling for the perceived positivity of the traits, indicating that these results didn't merely reflect a change in positive or negative attitudes towards the group. Put differently, even if the trait was perceived as positive, if it was not unique to humans, people felt it reflected their group less when reminded of death.
Super-interestingly (to us at least), when people humanized the in-group and were reminded of death, they had less thoughts of death (e.g., completing G R A _ _ with grape or grave).
From the perspective of terror management theory, culturally based worldviews protect people from an awareness of their own mortality. Because (in part) animals lack culture (or at least, have less culture, or are perceived to by most humans), and are not perceived as immortal, people distance from their animal nature when reminded of death. For instance, they agree less with an essay arguing that humans and animals are very much alike. The current research extends this need to distance from animality under mortality salience to the group level.
Mortality is salient in war and conflict. In turn, it is likely that these basic concerns contribute to the negative reactions between both sides. This isn't to say there aren't other reasons, however, but this work does suggest that death concerns play a role in this.
When reminded of death, we humanize our own group, and in turn, the out-group becomes less (relatively) human in our perceptions.