I've got a theory that I want holes shot in so I'll put it out here.

once you let the vampire in...

I think you're neglecting the relativism of morality. Those you view as lesser are often those that consider themselves the most pure - the KKK aren't a bunch of nefarious racists in their minds, they're Aryan purists out to unify and preserve the hegemony of the master race against the lesser species of God. The Nazis were only creating lebensraum for the Ubermensch. Aum Shinrikyo didn't spread nerve gas on a Tokyo subway because they were supervillians; they were hastening the coming of the apocalypse and the enlightenment of all mankind.

I think you're closer to a defensible argument if you state that those without a strong internal moral compass seek strong external reinforcement. "Lost" people "find" themselves in strict organizations all the time - it's an organizational trope. This is why shiftless adolescents are often shunted into the military.

There's nothing inherently bad about nationalism. Problems occur when it transcends morality. You're right - for those with a stunted internal compass, it's a lot easier to argue that torture is OK because The President Said So than to bulk up your conscience and decide you've been following the wrong leader. But that doesn't mean Scientologists join Scientology because they're shady.