I'm not going to worry about the origins of Hitler's antisemitism here, but just relate it to the "stab in the back" myth that many Germans embraced after WWI. At the time of the armistice that marked the Western defeat of Germany, the German army was still in France; it had not been defeated in the field, and it had not been driven out of enemy territory. This made it easy for some to believe that Germany had really been defeated by those in the German government who signed the armistice. These traitors subsequently created the Weimar Republic to replace the German Empire and ruled over Germany as a result of their treachery (the theory continues). The Social Democratic Party of Germany won the initial elections after the war, and, as you might expect in a nation with a large Jewish minority, some Jews found their way into the new government.

By the end of 1919, German right-wing writers were already accusing Jews, socialists, and even Bolsheviks of assisting in bringing about Germany's defeat in the war (there was a Bolshevik-aligned uprising in Germany that came into conflict with the SDP, but German writers nevertheless conflated the two). According to historians, this myth has no foundation; it is true that when the armistice was signed, German forces still occupied portions of France, but the German war machine was winding down and American forces continued mobilizing to join the war. German defeat was inevitable. Interestingly, this is one reason why the Allies decided to seek unconditional surrender in WW2. They did not want to again leave any doubt that Germany had been defeated outright.

Hitler, being a anti-semitic right-winger, naturally adopted this myth as part of his platform for governing. Conveniently, it lent credence to his message that National Socialism had to remove decadent elements from society (the socialists, the Jews) in order to make Germany strong again and rightfully restore a truly German leadership.