Jews were hated all through the middle ages and up to the Protestant Reformation. When Martin Luther started his church in Germany, he met with local Jewish leaders and expected them to join him since he had 'reformed' everything they didn't like about the Roman Catholic church. When they didn't, he was apparently very angry and wrote some really nasty things about the Jews. So 300 years later, anti-semitism was already a big thing in Germany.
Hitler didn't just hate Jews, he hated anyone who wasn't Aryan. But Jews had done pretty well in Germany and big banks and financial institutions were mostly owned and run by Jews. So they were a convenient scapegoat.
We are seeing kind of the same thing today with Trump and Hispanics. Hispanics aren't the problem, and down-on-their-luck seekers of sanctuary certainly aren't the problem. But Trump implies that they're the cause of all our trouble, that they are an 'emergency'. He doesn't plan to just exterminate them all (well, not yet) but Hitler didn't want to do that at first.