the fiend with an air of exultation
Nothing in the Creature's statements in the previous paragraph --
particularly given the stark alternatives he lays before Victor at its
end -- suggests that he is exulting at this point. Rather, the remark along
with the epithet convey Victor's intrinsic sense of superiority and of
his right to wield a power capable of affording satisfaction to an
underling. Whatever Victor may regard as "the duties of a creator," they
do not yet forestall him from calling his creature a "fiend" and
regarding him as "odious."
