I am writing an essay about the topic of isolation in Frankenstein. My first paragraph would be on how Victor isolates himself due to his pursuit...

Isolation is the perfect theme to write about for this book, as it certainly does come up multiple times and for each of the characters you mentioned. The trick about writing a thesis is to make sure that the examples you are providing all have something in common and while each of these do support the same theme, there needs to be more than just that to draw them together.

Let's look at the cause of the isolation for each character. Frankenstein isolates himself while pursuing his ambitions. Walton isolates himself while pursuing his ambitions (although not as much as Frankenstein does). The creature is forced into isolation because of how people react to him. By examining the reason behind each of their isolations, we see that Frankenstein and Walton are fairly similar.

So, how do you arrive at a thesis for a paper with these three examples? Consider something like, "Walton and Frankenstein isolate themselves as a result of their ambitious pursuits, and the creature's isolation is imposed on him; however, the effect of isolation on each of these characters is a negative one." That allows for you to move beyond simply noting each one is an example isolation and to talk about the consequences of it.