Why does Hellboy hate the Nazis?

Hellboy was summoned to the mortal realm by Rasputin who was working for the Nazis in a last-ditch effort to turn the tide of the war. Rasputin was trying to summon a demon who could destroy the allies. What he got was Hellboy. Years later, Hellboy’s adopted father was killed by Rasputin when he went to the arctic to find Rasputin and survivors of the project which had been formed to summon Hellboy. Dr. Bruttenholm was killed by Rasputin before his eyes.

Later, Hellboy also was able to see the details of his birth in 1917 and why he was summoned. He was repulsed by this and gained enduring hatred of the Nazis who had originally brought him into the mortal world with evil intentions. This is one reason that Hellboy ended up working for B.P.R.D. to fight evil forces to make up for the unholy purpose for which he was originally summoned.