Because he was pathologically antisemitic. No one has really been able to explain why Hitler was such a foaming-at-the-mouth antisemite. He himself explained it like this:

Once, when passing through the inner City [of Vienna], I suddenly encountered a phenomenon in a long caftan and wearing black side-locks. My first thought was: Is this a Jew? They certainly did not have this appearance in Linz. I watched the man stealthily and cautiously; but the longer I gazed at the strange countenance and examined it feature by feature, the more the question shaped itself in my brain: Is this a German? As was always my habit with such experiences, I turned to books for help in removing my doubts. For the first time in my life I bought myself some anti-Semitic pamphlets for a few pence.

And it apparently went from there. He started reading antisemitic literature and soon developed his own theories that surpassed most contemporary antisemitic writing by several lightyears. He accused the Jews of: producing smutty literature, dominating the press, selling German girls into prostitution and corrupting the honest German workers by insidious social-democrat and communist propaganda. He called parliamentary democracy a tool in the hand of the corrupt and false Jewish race.

He called the Jews sinister, evil, Satanic, like a maggot in a putrescent body, a moral pestilence, a bacillus which is the solvent of human society, parasites, a gang of public pests, vipers, criminals, leeches sucking the blood from the pores of the national body,

Let's have some more excerpts from Mein Kampf:

If the Jews were the only people in the world they would be wallowing in filth and mire and would exploit one another and try to exterminate one another in a bitter struggle

And this:

Economically he brings about the destruction of the State by a systematic method of sabotaging social enterprises until these become so costly that they are taken out of the hands of the State and then submitted to the control of Jewish finance. Politically he works to withdraw from the State its means of susbsistence, inasmuch as he undermines the foundations of national resistance and defence, destroys the confidence which the people have in their Government, reviles the past and its history and drags everything national down into the gutter. Culturally his activity consists in bowdlerizing art, literature and the theatre, holding the expressions of national sentiment up to scorn, overturning all concepts of the sublime and beautiful, the worthy and the good, finally dragging the people to the level of his own low mentality.

Historians as well as psychologists have tried to ascertain where all this hyperbolic hatred stemmed from, but there is really no satisfactory explanation for Hitler's extreme and sustained antisemitism, which ultimately led him to commit genocide on an industrial scale, dragging the German people down with him into that abyss of mass murder. Sure, there was antisemitism in Germany in the twenties and early thirties, but nobody was advocating the eradication of an entire people.

After WW1 the general public consensus in Germany was that the war had not been lost by their soldiers. While it is absolutely correct to say that German soldiers in this period were some of the finest, the reality remained that the German people needed to find someone or something culpable to blame their loss. Though Hitler would visit some of this blame on the leaders of Germany- while Hitler languished in a hospital away from the war which had given him identity and purpose as a person, suffering from exposure to mustard gas which would temporarily blind him and damage his throat, he would learn that the war was over and that the Kaiser had abdicated, leaving the Republic to fill in- Hitler also had a profound dislike of the Jewish race because they had no direct state. For someone like Hitler every race wanted for it's own country. Germany for the Germans, France for the French and so on, yet the Jews were in that same category as the Roma- they were a distinct ethnicity with no such thing as an autonomous state.

For Hitler this could only mean that they worked toward interests against that of the state they lived within. Hitler was very invested in this idea of the "monoculture" state. In Hitler's mind the Jews were at the forefront of a grand communist conspiracy intended to weaken the German State. This same belief was what propelled Hitler to get involved in politics and ultimately join the German National Socialists party, which he saw as an ideological opposite of communism.