This is probably one of the best examples of Hitler's political and personal cruelty.  In order to consolidate his own power and eliminate opposition to him, Hitler's design was to deem the enemies of Germany as less than human.  This dehumanization was what allowed the ease with which so many were tormented and killed.  It was not that Hitler ever said that "These are bad people."  Rather, he made the argument that individuals who were against him only did so because they were actually less than human.  Throughout his writings and speeches, the targeting of individuals of Jewish faith or homosexuals or gypsies was done so as arguing that the true standard of being a human being was not met by these groups of people, so keeping them is a crime against humanity.  This sad and twisted logic was embraced by so many.  Once the floodgates had opened in robbing people of their humanity though insult, dehumanizing comments and terms, and targeting a group of people, the abuse, intimidation, and elimination was easier to accomplish.

The answer is Hitler did not hate animals. He hated Jews with a passion.

Here are some examples of the experimentation that was conducted on the Jewish people and other people in the camps:

• Freezing-German soldiers were exposed to very cold and severe weather so experiments were performed to see just how cold a person could get and what injuries they would sustain before they died.
• Genetic experiments-Hitler dreamt of a Nordic or Aryan race (blue eyes, blond hair).
• Twins-Hitler had a fascination with twins. Twins were studied, tortured, killed and dissected for study.
• Internal Irrigation-Victims would have hot water forced into their stomach, bladder and intestines.

I have included a link that discusses many of the different kinds of experimentation that was conducted on people in concentration camps. It is very cruel and disturbing.

Good question.  Perhaps the root of it is in the part about permitting medical experiments to be carried out on prisoners in concentration camps.  The fact that a number of Nazis were in favor of animal welfare while being willing to systematically exterminate people is one of the things that makes the Holocaust so terrifying.  There have been many links between those who abuse animals and those who abuse children or other people, so the idea of someone who can have protective feelings towards animals and yet feel comfortable with trying to wipe a race of human beings from the face of the earth is utterly horrifying.

They even went so far as to decree that pets of non-aryan were not allowed so that animals could be preserved for foodstuffs.  But Aryans were still allowed to have pets.

If you are looking for the specific psychological mechanism that allows human beings to be this way, you are over my head!

This is because he did not think that animals were a danger to Germany.  He thought Jews were.  So he would "naturally" think that it was more acceptable to do horrible things to Jews than to animals.

In Nazi thinking, Jews were, of course, subhuman.  But then again, animals aren't human and Hitler did protect them.  But the difference is that Nazi thinking also said that Jews were (and always had been) trying to undermine German society.  So Hitler would have thought that Jews really deserved to have bad things happen to them while animals were innocent.