When they're not seen as humans but animals, vermin, etc. it becomes easy to commit such atrocities.

An excerpt from "On Freedom" by Timothy Snyder I'm currently reading helps define it some,

In Nazi propaganda, east Europeans were subhuman beasts. On Soviet posters, rich peasants were pigs. Russian television today takes this notion to a postmodern extreme: Ukrainians are to be exterminated because they are Nazis, Jews, gays, Satanists, ghouls, zombies, vermin. The next step is violence, the opposite of recognition of the Leib. When we torture or humiliate a body, we further objectify it. After the body has been degraded and the person humiliated, the killer finds the work easier to bear. Some enjoy it.

How he defines Lieb in the book,

The word Leib gets us to a new standpoint. The German language has two terms for “body,” Körper and Leib. The word Körper can denote a person’s body but also a “foreign body” (Fremdkörper), a “heavenly body” (Himmelskörper), the “racial body” (Volkskörper), and other objects thought to be subject to physical laws. A Körper might be alive, but it need not be (compare corpse). Stein says, “There can be a Körper without me, but no Leib without me.” The word Leib designates a living human body, or an animal body, or the body of an imaginary creature in a story. A Leib is a Körper, subject to physical laws, but that is not all it is. It has its own rules and so its own opportunities. A Leib can move, a Leib can feel, and a Leib has its own center, impossible to graph precisely in space, which Stein calls a “zero point.” We can always see some of our Leib, but we can never see all of it.