I am reading Giorgio Agamben's The Open: Man and Animal. In it he says that:

As is well known Heidegger constantly rejected the traditional metaphysical definition of man as animal rationale--the living being that has language (or reason), as if the being of man could be determined by means of adding something to the "simply living being".

I do know that Aristotle initially made that distinction to establish the "human" as a distinct and superior type of animal. Latter on, with Descartes, language was used to establish the "human" as a kind of being that is essentially different from animal altogether.My preference is Aristotle's 'man is by nature a rational animal' as this suggests the continuity between animal and human, no matter how great the differences between them.

Dehumanization refers to the reducing of the human to the animal: "animalizing" humans.The mirror of this is the violence against the non-human: the cruelty torture and violence that is directed towards animals in slaughterhouses, experimental laboratories and factory farms.

At this point we begin to see the connections between the slaughterhouse and the concentration camp.