To what extent was Hitler's aggression the main cause of WW2?
To some extent, the answer to your question depends upon what you are including in referring to "Hitler's aggression," but his actions and policies can be identified as the most directly specific causes of World War II.
Hitler's hatred of the Jewish faith, and all who adhered to it or appeared as if they might have Jewish ancestry, was first published in a letter written in 1919 and greatly expanded in Mein Kampf, published in 1924. However, he blamed a Jewish doctor for the death of his dearly beloved mother in 1907 and frequently used the image of "the Jewish parasite (or cancer), the Jewish poison, and the Jewish profiteer" in his campaign justifying genocide against the Jews.
At the same time, Hitler was mortified by the defeat of Germany in World War I, which he blamed on the Jews. He vigorously called for rebuilding of the German military strength and unity, so as to support expansion of the German area of influence as an indication of power and as protection against threats from others.
The struggle for world domination will be fought entirely between us, between Germans and Jews. All else is facade and illusion. Behind England stands Israel, and behind France, and behind the United States. Even when we have driven the Jew out of Germany, he remains our world enemy.