The roots of European antisemitism are very deep. Mostly it goes back to when The Third Lateran Council made collecting interest on loans grounds for excommunication (a change backed up by Pope Clement III.) since Christians could no longer make money as bankers or moneychangers (or goldsmiths and jewelers, since those trades were closely related at the time) Jews became the bankers of Europe.
And who loves the guy you owe money too?
Jews were also feared because they worshiped differently, spoke their own language, and generally kept to themselves. Long before Hitler pogroms against Jews were common, and at one point or another a "Christian" monarch threw all the Jews out of just about every country in Europe (except for his personal physician and banker, of course.)
For years, "blame the Jews" was the easy thing to do. Plague kills half the town? Jews. Well dries up? Jews. Daughter runs off to follow Paganini on tour? Has to be those darn Jews.
Hitler, who was already a bit of a nut on the subject, just gave the German people someone to blame for their problems. The loss in the Great War, the failures of post-war governments, the Treaty of Versailles, hyper-inflation, all that wasn't the fault of good Germans! No, it was those alien Jews again.
There is some evidence that mass genocide wasn't the original plan, instead Jews would in fact be deported en mass to slave labor camps in the occupied Soviet Union there to be worked to death. But when the Wehrmacht failed to win a quick victory, a final solution was needed.
Hope this helps, there are many excellent books on the subject.