I do not think that it had to do with him being a product of incest. It was most likely because of his early home life.
Explanation:
Adolf Hitler was born to Alois Hitler and Klara Polzl. The relationship between his parents is thought to be that of first cousins once removed. His aggressive behavior can be linked to his childhood days when he always got into trouble with his father who was short-fused. This likely affected his mental health for he began to withdraw from social activities while in school.
He would rather delight in relishing scenes from the Boer War. Losing his parents at an early age and having to hustle for the early parts of his life in hostels must have also contributed to his becoming hardened and the aggressiveness that followed.
