There are many theories regarding Hitler's hatred of the Jewish people. One of these reasons is mere jealously. The Jewish people were not hit hard during the Great Depression and Germany was hit the hardest. Hitler also blamed Germany's defeat in WWI on the Jewish people. Also, some blamed the Jewish for the "Treaty of Versailles" which was very damaging to the Germans.
The idea of the Aryan German race was also to blame for Hitler's hatred of Jewish people because they were not part of the ideal white race that Hitler envisioned.
In addition, Hitler supposedly had a bad experience as child with Jewish people and this is when the hatred was seeded.
These are just a few of the theories why Hitler hated Jewish people so much. There are many many more.
He didn't have to work that hard to justify it at that time in Germany. Jews had been the "outsiders" for centuries, had been economically successful, worshipped and lived differently, and tended to marry among themselves. They were an easy target as the cause of all of Germany's ills.
Hitler himself did not just make up his hatred so he could be politically powerful, he believed it too. He had grown up and experienced life in a time where anti-Semitism (anti-Jewish feelings) were common and openly expressed, and this rubbed off on him. He blamed the Jews for the loss in World War I, and for the economic hard times of other Germans in the years after the war.
Hitler used a variation of tactics to justify hatred of the Jewish people. He had physicians identify specific features that the majority of the Jewish population had in common. Some of the features that he had identified were head shape and circumference, brow distention, hook nose, and length of the nose.
Hitler was also aware that many of the Jewish people had booming enterprises. He resented this and used it to his advantage. The country was still suffering from the damages caused by World War I. He informed the population that while many of Germany's best men died fighting for their country, the Jews had stayed home and milked the economy and become rich.
Hitler was smart enough to recognize that power would be obtained by uniting his country in hatred. He rationalized that he would be doing Germany a favor by ridding it of the vermin. He considered the Jews to be vermin.