First of all, Adolf Hitler was not a German. Though he spoke German, he was from Austria. It is said that it is not unknown who is Hitler's father. There is a doubt that he actually might be a Jew.
In Austria, Adolf Hitler had many good Jewish Friends. In the process of growing up, it is said that many Jewish friends helped him. Adolf Hitler wanted to become an artist but he was mediocre. When Hitler went to an Arts School, he was rejected there and advised that he can not be a good artist.
There is a line of argument that Hitler thought that Jewish people did not want him to be Artist as the board of admissions was all Jewish. So, there could be that Hitler wanted to take this revenge in its worst form.
It is said that when Adolf Hitler attacked Austria, he ordered to destroy completely his native village where he was born and spent his childhood. The story is that he feared that his father might be a Jewish one and he hated his father, the Jewish and any attachment to his father and father's home.
As a mode of justification of killing of Jewish, Hitler developed the "Aryan Race Theory". Hitler considered that the Germans - Aryans were the Pure Race and the people with the best race. As per Hitler, Jewish were the people with inferior race. Jewish Race did not deserve to live and Hitler blamed Jewish for the defeat of Germany in World War I (WWI). These are the explainable but possible reasons why #Hitler committed the most heinous crime in Human History. For Hitler, the Aryans were the Master Race and Jewish People did not fit into his "Concept of Master Race".
As per the report on Daily Mail, an English News paper, it might be that it was convenient for Hitler to put a blame on Jewish. The Daily Mail Report says, quoting a history book, "At the time almost half of all German private banks were Jewish owned, the stock exchange dominated by Jewish stockbrokers, almost half of the nation’s newspapers were Jewish run as were 80 per cent of chain stores. It became fashionable to decry the loss of the war on Jewish financiers."
It is said that Hitler was always against Communism and blamed Jews for Russian Revolution under Lenin in the decade of 1910s. Many Communist philosophers had Jewish Links.
Therefore, there were possibly many reasons why Hitler was against Jews. But, whatever the reasons might be, he indulged in undeniably the most atrocious crimes in Human History.