In his first letter to his sister, Walton writes of his journey to Archangel from St. Petersburg . Frankenstein is rich with religious metaphor and these city names are curious in light of this, referring to both an important saint and a rank of angel. It is possible to draw a parallel between this route and Walton’s roles within this metaphor as an archangel and St Peter.
Walton’s role as an archangel extends to the metaphor of Frankenstein as God and the monster as Lucifer . Archangels are often regarded as the most revered of angels, God’s most powerful servants and protectors. Walton is something of a protector to Frankenstein, taking care of him in the weakness of his last days. In his final words Frankenstein dreads leaving his mission, (to kill the monster), in the hands of another . Walton recalls this when facing the monster  and while he does not end the monster’s existence he witnesses the monster’s declaration of it’s intent to die . Walton as the archangel, to Frankenstein’s God, thus witnesses the expulsion of Lucifer that is the monster.
St Peter is the guardian of the Pearly Gates, the gateway to Heaven. He therefore decides who enters Heaven. Walton listens to Frankenstein’s tale, and hears his “confession” before Frankenstein passes. Walton is taken by admiration and sorrow for Frankenstein , perhaps in this way redeeming the scientist of his vanity and early self-importance. Walton is also present to hear the monster’s confession, the admission of the guilt and self-loathing the monster feels . Walton here parallels St Peter, standing at the gateway between life and death for these characters. He also witnesses the judgement Frankenstein and the monster have on themselves, the recognition of their faults and their resolution of it, however unsatisfying those resolutions may seem as both die (or leave to die) without a final stand off with each other.
It is also fitting that the route from St Petersburg to Archangel runs north, perhaps symbolising ascension for both sorrowful characters.
 “…the post road between St Petersburg and Archangel.” Page 5
 More info on religious metaphor here: http://www.123helpme.com/view.asp?id=14625
 “Yet I cannot ask you to renounce your country and friends to fulfill this task…” Page 269
 “…duty of obeying the dying request of my friend in destroying his enemy…” Page 272
 “…I shall collect my funeral pile and consume to ashes this miserable frame…” Page 276
 “…untimely extinction of this glorious spirit? What can I say that will enable you to understand the depth of my sorrow?” Page 270
 “But it is true that I am a wretch.” Page 275