Psychology has spent the entirety of the post-war era addressing just this. For example, right in the aftermath of WWII Adorno et al. began researching authoritarianism and fascism (which is where the F-scale, or Potential for Fascism Scale, originated) amongst the general population.
It was found that, regardless of background and culture, a frighteningly high proportion of randomly selected participants scored highly on the f-scale and tended toward ethnocentrism. While the implications of this have been re-dissected and discussed greatly over the years (and further improvements and conclusions made) the fact remains that as a society we tend to want to isolate such individuals (Hitler, Mao, Myra Hindley) as 'monsters'; something unique and alien to the rest of humanity.
Unfortunately, research bears out that given appropriate environmental and cultural backing (for example an authoritarian ethnocentric upbringing, cultural acceptance etc) that many of us are equally capable of such atrocity. Stanley Milgram's research in this area, especially, confirms this latent streak amongst the general population in a willingness to follow orders and impose pain and punishment to fatal levels simply upon pressure of instruction.
Frightening revelation, but an important one nonetheless as it's only by understanding and confronting our true selves that we can shape and improve our behaviours.
I don't know much about Naziism per se (aside from its fascistic root) but I thought this might provide an interesting aside for you on the matter.
