The "story" and the "blot," of course, refer to Monygham's betrayal of innocents during his torture. But this is also another parallel between Monygham and the Devil (who also has a "story" and a "blot"). I.e., if the People realize who they are dealing with, they will have "escaped his influence."

Purely on the psychological level, note the blindness engendered by Monygham's idealized disgrace. It is not necessary for Nostromo to know the doctor's history for him to be worried here, since the doctor has just boasted of his own callousness with regard to Hirsch.