The fact was that after most of the western world had abolished slavery for over one hundred years, the jewish people were dehumanised to a level that was disgusting. It was a crime to kiss or associate with the impure jewish people, they were forced labour in the Ghettos, and later in the concentration camps. In which utility was the only reason to keep jews alive, not their humanity.
Secondly, the fact that this was in many respects industrialisation taken to its grotesque and horrific conclusion. Namely that a factory process was introduced just for the murder of Jews, from experimentation with the cheapest and least morale sapping way of doing it, having tried firing squads and then several different kinds of gas for use in gas chambers. More than this there was a construction of vast incinerators to allow the bodies to be disposed of as quickly as the Jews could be killed.
Finally the fact, that at the very last moment, when it was clear that the war was lost, the Nazis attempted to destroy all evidence of their horrific crime against humanity, denying the survivors of any reminder of their loved ones. So much that we can only estimate how many died, and never know for sure, as so much of the physical evidence that these people ever existed was taken away from them.
The Nazis did not just kill these Jews, they humiliated and dehumanised them, they did not even murder them, instead they created a system so impersonal and abstract that it can only be termed an extermination, and then lastly they tried their very best to wipe the fact that they have ever existed.