The concept of the banality of evil originated with Hannah Arendt and although she eschewed the term philosopher, the "banality of evil" is a concept best employed as a philosophical way to understand history, not as a tool to interpret history.

The main problem with the banality paradigm is that it fosters a conceptualization of these perpetrators into a Manichean division between the apolitical bureaucrat and committed ideologues. This is something of a false dichotomy and falls apart when one examines the actual biographies of the individuals involved in the Holocaust. Michael Wildt's study of the RSHA, An Uncompromising Generation, shows how the SS's organization combined technically-minded bureaucrats who shared the radical racialist vision of National Socialism. Wildt demonstrates that both radicalism, which often eschewed formal logic and Enlightenment ideals, and bureaucratic professionalism worked synergistically within the RSHA rather than being at odds with each other. The extermination camps might have been designed rationally, but a number of accounts from survivors report that the actual operations had some of the very same cruelties and alcohol-fueled sadism that characterized the Einsatzgruppen.

That being said, Arendt shouldn't be put in the dustbin- I would argue Eichmann in Jerusalem is an important work to use when coming to grips with the history of genocide. Popular cultural constructions of evil often build upon othering and erecting between the nominally rational audience and the irrational acts of evil characters. Thus Nazis on screen and in cinema frequently behave in a monstrous and irrational manner in a way that suggests this was the totality of their lives. Arendt acts as a check on this othering.

If you're interested, Dan Stone's Histories of the Holocaust has a good chapter on the decision-making process and does an excellent job of surveying how historians have viewed the perpetrators.

Assuming you meant to say that Arendt's concept is more applicable to "lesser" Nazis than high ranking officers. The idea behind the banality of evil is that is generally and usually always applies to those existing within a certain ideology. I don't think it's any better applied to guards than it is to Eichmann, but they're both applicable.

Read Eichmann in Jerusalem if possible. Origins of Totalitarianism first if you want a decent history and thorough understanding of Arendt's ideas.