Hitler believed that they were the superior race, and the only ones that deserved to live in what God created, and some believe Hitler was mentally insane, which could be another reason.
Hitler wanted to create a master race with the racially pure German aryans at the top of the pyramid reaping off the slavery of others- thus it is called "National Socialism." They were to be nationalistic and socialist and at the same time.
The Poles, Slavs, Africans and Asians would have been exterminated or enslaved because of Hitler's racism.
Homosexuals would have been because he was a homophobe (as most Christians of that era were). The disabled were killed because they were deemed to be drain on the country.
Christian priests were more likely to be complicit in the crimes of the Nazi regime than victims of the regime, a few that opposed the Nazis were killed but most Christian clergy were helping or at least not opposing the Nazis.
The JWs didn't believe they owed any loyalty to any worldly government which meant they weren't willing to help the Nazis and thus they became enemies (also whilst Hitler was a devout Christian he was probably somewhat intolerant of minority Christian sects).