It's funny you mention that, I'm reading Mein Kampf at the moment, so who better to explain the importance of it than Hitler himself?

Throughout the book he mentions that his dream was attending art school since he was a child and, while he did show a serious interest in architecture as time went on, he still dreamed of himself as an artist (in addition to utterly refusing to become a civil servant).

He mentions being, essentially "crushed" by his art school rejection, it seemed to catch him by a great deal of surprise. He was also unable to attend architectural school in part because he was kept from finishing realschule because of a disease he'd gotten as a child.

Did his anger at being kept from art-school however turn him into the genocidal dictator he became? I would say no or at least not entirely. In Mein Kampf, it seems the real turning point would be the poverty he witnessed and experienced in Vienna as really radicalizing him. He goes into great detail describing his experience in Vienna and from the pure "tone" of how it was written, it seems to be the real turning point in which he was radicalized it. In the book, he describes it as follows:

"The life which I had hitherto led at home differed little or not at all from the life of other people. Carefree, I could await the new day, and there was no social problem for me. The environment of my youth consisted of petty-bourgeois circles, hence of a world having very little relation to the purely manual worker. For, strange as it may seem at first glance, the cleft between this class, which in an economic sense is by no means so brilliantly situated, and the manual worker is often deeper than we imagine. The reason for this hostility, as we might almost call it, lies in the fear of a social group, which has but recently raised itself above the level of the manual worker, that it will sink back into the old despised class, or at least become identified with it. To this, in many cases, we must add the repugnant memory of the cultural poverty of this lower class, the frequent vulgarity of its social intercourse; the petty bourgeois' own position in society, however insignificant it may be, makes any contact with this outgrown stage of life and culture intolerable.

Consequently, the higher classes feel less constraint in their dealings with the lowest of their fellow men than seems possible to the 'upstart.' For anyone is an upstart who rises by his own efforts from his previous position in life to a higher one. Ultimately this struggle, which is often so hard, kills all pity. Our own painful struggle for existence destroys our feeling for the misery of those who have remained behind. In this respect Fate was kind to me. By forcing me to return to this world of poverty and insecurity, from which my father had risen in the course of his life, it removed the blinders of a narrow petty-bourgeois upbringing from my eyes. Only now did I learn to know humanity, learning to distinguish between empty appearances or brutal externals and the inner being."

However when discussing the rise of the Nazis, it's important to try and avoid "simplifying" things, of just pointing to one event in history or aspect of the Nazis and just saying "So this is how it all started!" or imagining that one specific event made a young Hitler wake up one day and decide he hated the Jews.

Hitler, and Fascism as a whole, was very much a product of the changing environment of their time and their views were shaped as much by their personal experiences as well as the context of the world around him. For example a number of the socialist commanders of the German Revolution of 1918-1919 were Jewish, which helped fuel the Nazi claims of "The Jews" as having "Stabbed Germany in the back" during World War One and played a role in perhaps developing the virulently anti-semitic attitudes of the Nazi Party. There was also a kind of "German Diaspora" that provided some motivation for the Nazi's imperialistic foreign policy.

So, in essence there likely isn't any one event that "flipped a switch" and set Hitler out onto his destiny, but rather a series of events, experiences, and changes in the world that shaped his views and the views of the Nazi party as a whole. If you had to point to one really early event in his life, though, it'd probably be between his life in Vienna, his experiences in the trenches of WWII, and/or possibly his meeting with Ernst Rohm.