Why does "Tom" begin traveling to St. Louis support or refute Twain's position on nature versus nurture?

I don't think the book supports either claim. On one hand, one might argue that "nurture" is responsible for Tom's disposition; being raised white and rich led to him becoming a spoiled, cruel young man. This argument would further assert that had "Tom" remained a slave and received a more austere upbringing, he likely would have grown to be meek and docile, as Chambers did. On the other hand, it might be suggested that it was Tom's inherent "nature" which determined the kind of man he would become; that his laziness and dishonesty are in fact manifestations of his inborn slave qualities. In fact, Roxy later suggests precisely this, claiming that it's Tom's "blackness" that causes his cowardice.

The argument that Tom's laziness and dishonesty are a result of his "nature," can be undermined. Unlike his contemporaries, Mark Twain refuses to portray his black and slave characters as inherently dishonest or lazy individuals. Rather, as Twain frames it, acts of thievery by slaves against their masters are a form of defiance and rebellion.