What is Machiavelli's primary purpose in writing this
passage?

To convince readers that, to hold a conquered republic, a prince must ruin it or live there.

i believe it is the first choice (1.)

because the major theme is(“both the wild and civilization have rules.”)

and also as u said,( based on what buck learns within the call of the wild, "both the wild and civilization have rules".this is true of humans within society as well.