Interesting question. The term was coined by Hannah Arendt, a critic and historian who was discussing how everyday Germans morphed into Nazi monsters, but when put on trial, seemed so ordinary and commonplace, not like monsters. One implication is that evil deeds are done by ordinary people, even ones that lead quiet, repressed lives like Emily Grierson. She seemed odd, yes, but capable of the horror that we discover at the end? It comes as a shock because everything just seemed on the surface to be a bit eccentric and odd and sad, but nowhere near what the truth of the matter was. Emily is short and fat and obviously "different" and just a bit sad. But we never see her as someone capable of murder and corpse abuse--until the end. This is one of the great examples of Southern Gothic style--one that Faulkner used repeatedly to suggest the unrest and disharmony and extreme emotional turmoil hiding behind the facade of Southern aristocracy and graciousness.