Just because someone is fanatic about their beliefs or has extreme beliefs doesn't mean they're stupid or unintelligent. Far from it actually.
The majority of the men who led Einsatzgruppen/Einsatzkommando units in Poland and the USSR had doctorates or PhDs. One of them even had two PhDs. These were highly educated men yet they could participate in or oversee the ruthless execution of thousands of Jewish men, women and children day after day, massacre after massacre.
That is what's wrong with any ideology or belief system. It can turn even the kindest, most loving, rational and sane people into monsters, zealots and cold blooded killers.
Same goes for Goebbels. Take any man and have him genuinely believe in Nazism and its doctrines and see if he doesn't support, condone or perform horrid acts against fellow humans or in their eyes "untermensch."
