Having in the previous letter (I:L1:4) already employed language associated with Milton's Satan, here Mary Shelley directly echoes his description:
Deep scars of thunder had intrencht, and care
Sat on his faded cheek, but under brows
Of dauntless courage, and considerate pride
The author's purpose seems not to be one of branding this crew with a diabolic association (though it is true that they will later become united in rebellion against their master), but rather this early on in the novel to plant motifs that will serve as unifying structural and thematic devices as Mary Shelley begins to interweave multiple narrative lines. In this case the association of the heroic and the Satanic will provide a perspective in which the reader will later frame both Victor Frankenstein and his Creature.