how does the end of the novel justify the concentric levels of narration introduced at the beginning

Basically we have to consider that Victor, the first-person character narrator, can't be around to narrate his own death. I think Laertes tried that in Hamlet; it didn't work. Shelly needed some way of including his death in the story hence including Captain Walton and his letters to his sister.

Narrator the end of the beginning.

Thank you