1) They see Jews as a foreign 'inferior' race living among 'proper' Europeans.
2) They see Jews as holding influential positions in European and American society.
3) They see Jews as worming their way into European society in order to destroy Western civilzation.
4) Hitler saw the European race, in particular the Germanic race, as the highest stage in human evolution and the Jews at the bottom, under the blacks.
5) In his book 'Mein Kampf', Hitler proposed that the Jews were jealous of the Germanic race and so sought to corrupt German culture.
Antisemitism was already present in Germany before Hitler. Ever since the Middle Ages, Jews left their native land (by now under Muslim rule) seeking asylum and fled to Europe where some Christian countries took them in, notably Germany, Poland and Russia. The Jews didn't quite fit in. They kept their own religion, their culture and language. Some made their way to positions of business power or influence in the media. Prejudice against them existed down through the ages. Hitler used this prejudice to stir up racist hate.