This is one of the reasons why I selected "Humans evolved from Apes"
Dehumanization is a psychological process whereby opponents view each other as less than human and thus not deserving of moral consideration. Jews in the eyes of Nazis and Tutsis in the eyes of Hutus (in the Rwandan genocide) are but two examples. Protracted conflict strains relationships and makes it difficult for parties to recognize that they are part of a shared human community. Such conditions often lead to feelings of intense hatred and alienation among conflicting parties. The more severe the conflict, the more the psychological distance between groups will widen. Eventually, this can result in moral exclusion. Those excluded are typically viewed as inferior, evil, or criminal.[1]
We typically think that all people have some basic human rights that should not be violated. Innocent people should not be murdered, raped, or tortured. Rather, international law suggests that they should be treated justly and fairly, with dignity and respect. They deserve to have their basic needs met, and to have some freedom to make autonomous decisions. In times of war, parties must take care to protect the lives of innocent civilians on the opposing side. Even those guilty of breaking the law should receive a fair trial, and should not be subject to any sort of cruel or unusual punishment.
However, for individuals viewed as outside the scope of morality and justice, "the concepts of deserving basic needs and fair treatment do not apply and can seem irrelevant."[2] Any harm that befalls such individuals seems warranted, and perhaps even morally justified. Those excluded from the scope of morality are typically perceived as psychologically distant, expendable, and deserving of treatment that would not be acceptable for those included in one's moral community. Common criteria for exclusion include ideology, skin color, and cognitive capacity. We typically dehumanize those whom we perceive as a threat to our well-being or values.[3]
Psychologically, it is necessary to categorize one's enemy as sub-human in order to legitimize increased violence or justify the violation of basic human rights. Moral exclusion reduces restraints against harming or exploiting certain groups of people. In severe cases, dehumanization makes the violation of generally accepted norms of behavior regarding one's fellow man seem reasonable, or even necessary.