Yea Hitler wrote in Mein Kampf that he hated Jews and Communism so it should've been taken very seriously since a person like him(German nationalist and fanatic) became a chancellor. I don't who expected different behavior than what was written in Mein Kampf. Holocaust and WW2 War could've been prevented if Allies(US, UK and France) invaded Germany in 1933-1939 time period. But they expected Hitler to be European shield against Communism and geo-politic partner.