Why did Adolf Hitler become a total vegan some time in 1938 and before even Kristallnacht?
He wasn't vegan. Some historians say that he was vegetarian at some point in his later life, but that is not a given either.
No one really knows why Hitler became a vegan in 1938. It may be because he was such an animal lover. https://en.wikipedia.org/wiki/Adolf_Hitler_and_vegetarianism