Question

Hitler's hatred of the Jews began with what assumption

Answer 1

There are a few reasons that could have contributed to this hatred he developed towards Jews :
-the hatred of Jews was very common in the city.
-a myth that Hitler believed. The myth was that Jews and communists had betrayed the country.

Answer 2

Hitler lived in a city called Vienna with a large Jewish community where he lived from 1907-1913. Vienna had a mayor that was very much anti-Jews, where the hatred for Jews was very common.