Why might people have been horrified when they found about the full scope of the holocaust?

I would have to say that the way the Jewish people were treated.
At the beginning they were removed from their homes and humiliated, their possessions taken away. Then they were sent to interment camps.
The "lucky" ones were the ones who were taken to the work camps.
The fact that they were starved, beaten, physically, mentally, emotionally abused, their heads shaved, left in very little clothing was horrifying.
The fact that Hitler murdered the sheer number of people: Jewish, mentally & physically "damaged", was also horrifying.
The fact that it was hidden for so long, in that the worst camps were not shown to many, is equally horrific.
Hope this helps.