Once animal exploitation was institutionalized and accepted as part of the natural order of things, it opened the door to similar ways of treating other human beings, thus paving the way for such atrocities as human slavery and the Holocaust.
Calling people animals is always an ominous sign because it sets them up for humiliation, exploitation, and murder. It is significant, for example, that in the years leading up to the Armenian genocide, the Ottoman Turks referred to Armenians as rajah (cattle).
This use of animal terms to vilify and dehumanize the victims, combined with the abominably degraded conditions in the camps, made it easier for the SS to do their job, since treating prisoners like animals made them begin to look and smell like animals.
Throughout the history of our ascent to dominance as the master species, our victimization of animals has served as the model and foundation for our victimization of each other. The study of human history reveals the pattern: first, humans exploit and slaughter animals; then, they treat other people like animals and do the same to them.
Those who kill “humanely” often contend that their victims suffer minimally or not at all. This contention helps ease their guilt and makes the continuation of the killing more acceptable.